The Central Idea: A small game about a small group of people who meet in an abandoned city in the middle of a desert. Mostly told in flashbacks about how they came to reach the city. I'd be wanting to try come up with a GMless 'big reveal' technology to go with this one, since I'm vaguely thinking that some of the people may actually be ghosts or hallucinations and who is real wil be hidden information of some sort.

A Skeleton of a Mechanic:

We have a fairly large amount of deck manipulation to do at the beginning of the game. Start with a regular 52-card deck of playing cards, no jokers, and sort it out into three packs: first, the aces, second, the face cards, and third, everything else. While keeping everything face-down, pick one ace. Then shuffle the remaining aces into the face cards and deal three cards from that pack onto the ace. Randomly distribute those four cards to the players, of which there will always be four. This is their Identity Card. Then deal out the spot cards to each player, who should then insert their Identity Card randomly somehwere near the middle of the stack. The 12 remaining face/ace cards remain separate for now. Each player draws the first three cards from their pack to form a starting hand.

Each player picks a name for their character in the City, and each one in turn will start by narrating a scene about why they went to look for the City and who was with them. When narrating a scene, you play the lowest card in your hand face up. In the first scene for each player, three more characters are introduced, which are who the other players are in the narrating player's flashbacks. [And they should play all four of their characters with essentially the same personality, just in different circumstances.]

During the narration, a person holding a card higher than the scene card may play it as a trump/veto. He asks a question (which I think is going to be determined by the card's suit) that will force a reevaluation of the scene. That card becomes the new scene card.

Since I'd like to avoid pure parlor narration and because I want play options for all situations, there's a second card play mechanic that works in the other direction. You can play a card less than the scene card when you, during the roleplaying of the scene, get another character to do something to your character (with that something being determined by an oracle based on the card you're trying to play). This card doesn't replace the scene card. I think that players can only play one card per scene.

When the scene ends, everyone who played a card draws a new one from their pack. If that card is their identity card, they should announce that they have had an Epiphany, state the suit of their identity card, and then set it aside face down. Then they should put their current hand at the bottom of their pack and draw three cards from the stock of face cards and aces. Any of those cards that get played should be placed aside, face up in a tableau so that deductions can be made as more of them appear.

I think there will also be a second oracle to determine the broad nature of scenes after each player's first introductory one.

The meanings of the identity cards are as follows: Aces are real people, Kings are ghosts, Queens are benevolent imaginary friends, and Jacks/Knaves are malevolent spirits.

The 'trump questions' by suit, I think are:
Hearts: Hidden feelings ("You didn't let on, but what just happened deeply hurt you. Why?")
Clubs: Outright Deception ("That's not what really happened, was it?")
Spades: Deception by Omission ("What aren't you telling us?"}
Diamonds: Ulterior Motives ("What was your real agenda here?")

(Within the frame, the scene starter is telling the story to the other three characters as they shelter in the City, and the trump/Edge Over jumps back into the frame where another character calls BS on the narration, forcing a reevaluation.)

The Endgame triggers when someone runs out of cards.  If anyone hasn't had an Epiphany at that point, they get to search their pack for their Identity card and look at two cards from the aces/faces stick, not revealing them to anyone else.  Then, everyone writes down who they can trust.  It really only matters who the real/Aces trust, but everyone has to write things down to avoid information leakage, and the end result is based on whether the real people chose to trust the right people.  (I'm not, at this point, sure what this means with respect to Ghosts/Kings.  Possibly they write down who they are loyal to, based on their experiences during play, and the trust/don't trust there is base on matching?)

This sounds pretty cool. I really like the idea.

I might be missing something, though, but it seems like choosing who to trust would be mostly random chance. The players don't know what their own Identity Card is until part of the way through the game, right? So they might not be playing that part until later, and when it comes time to choose who to trust, the human has no real information to go on.

Again, though, I might have just misunderstood when reading it. I definitely do like the basic idea.

You get your information through deduction.  The Identity cards are set aside, face-down, after each epiphany.  You know everyone's suits by the end of the game (when they had their epiphany).  And you keep track of the ace and face cards as they're played (they get laid out separately in a 4x4 grid).  So you should at least be able to narrow down the possibilities for a lot of the players.  (You also know your own identity card, which may help in that narrowing down, and you may well have a few face cards still in your hand as private extra inforrmation.)

So, I've started my first draft.  I'll probably switch to working on the oracles tonight, where I'm leaning towards the 13 slot version for scene types and still not sure about the Edge Under one. I guess I'll see if I can come up with 48 interesting and achievable reactions.  I'm trying to engineer a strong bias towards intense melodrama here.  (The traits don't do anything in actual play, but they should help steer players to the right sort of characters, as well as giving them something solid to hold as common between the four 'versions' they'll be playing).  After that, I'll write the meat of the game, the "Scenes" section, and the Endgame one.  Still uncertain about how to mechanically end scenes fairly.  Anyhow, first draft of flavor text, introduction , and set-up section follows:

Most people say there is no city in the middle of the desert, that there never has been.  But the desert wasn't always a desert, and the legend of Urbe is too prevalent, too compelling to be pure imagination.  So you set out, with an expedition of a few dozen, to seek it out.
Of them,only you remain.  But it was worth it.  You have found the city, still glorious in its sand-swept ruin.  And at the center, in a tumbled-down park, is an oasis, with water to sustain you at least for a while.
You are not alone.  There are three others, strangers but somehow familiar, gathered with you at the oasis.  Each claims to be from an expedition like your own, but you doubt you can trust them.  Your rivals may have been close behind you, and may be your only hope of rescue, but four reaching the waters at once?  It is scarcely to be believed.  And you've been seeing things, among the ruins, out of a corner of your eye.  This place is haunted.

Introduction
A Trick of the Light is a roleplaying game for four players.  At least one of them is playing the role of a real person who traveled to the ruins at the center of the desert.  The others may be real humans, ghosts of people who died along the way, or ancient spirits, good or evil, that have resided in the ruined city for generations.  To pass the time as you wait for rescue, you have chosen to tell stories about the journey that led you here.  As you tell and listen to these tales, you will slowly gain information, about your self and your companions.  In the end, if you are real and alive, you will need to make a decision about who can be trusted.  If you are a spirit, you will need to lead the living to a decision, good or bad depending on your nature.  And if your are a ghost, you will need to name your killer, and hope to serve vengeance or support the innocent.

Set-up
First, each player should create their main character.  To do that, they should pick a name, a role in the expedition to the desert, and three personality traits.  These traits should all be strong, dynamic, and specific, and should balance between positive and negative traits.  A few examples of good traits are: Zealous, Foolhardy, Faithful, Craven, Lustful, Servile, Motherly, Short-tempered, Mournful, Rebellious.  Try to avoid passive traits, like Shy or Thoughtful, and overly abstract traits, like Loving or Cautious.  Also try to make sure that you have three different traits rather than a single trait with two or three names.
Second, prepare the deck.  Start with a standard deck of playing cards without jokers.  Find all four Aces and set them aside, and separate the remaining cards into two stacks, one with all of the face cards (Jack, Queen, King) and the other with the spot cards (2-10).  Put one of the Aces, at random and face down, into a stack in front of you.  Shuffle the remaining aces into the stack of face cards. Then put the top three cards, still face down, on top of the Ace you just selected.  Shuffle these four cards and then give one, still face down, to each player.  This is their Identity Card.
Next, shuffle the stack with the spot cards and deal them out to each of the players, each of whom should end up with nine cards.  Each player should then draw the top three cards of their to form their starting hand.  They may look at these cards.  Then Each player should, without looking at it, shuffle their identity card into their stack.  Now you are ready to play. 

Okay, finished up the first draft. A fair number of tables, so it might look ugly in this format, but here's the rest of the document:

The Introduction Mini-Scenes
Starting with the youngest player and going clockwise around the table, each player should narrate a short scene introducing their expedition. This should take place at the very edge of the desert, as they leave civilization behind in search of the lost city. This scene should establish exactly what the members of this expedition are hoping to find by reaching the city, and establish what their role was in the expedition.
Each of the other players should introduce a version of their character as a member of the expedition. They should pick a name that is a variant of their main character's name, and fill out one of the lines on their character record to remind you of your character's name and role in this set of flashbacks. Sub-characters should be different in circumstances from the main character, and it is recommended that at least one of the sub-characters be of a different gender. However, the three central personality Traits that define your character should all be common across the main character and all four sub-characters. No cards will be played during these mini-scenes, and they should be kept short and fairly simple.

Scenes
After the four introduction scenes are complete, regular play begins, again with the youngest player and moving clockwise. To start a scene, the player whose turn it is should play a card (henceforth known as "The Scene Card") and consult the chart and begin narrating a scene that fits that description. The first part of the Scene Type column is the actual scene type, and after it, in parentheses, is a suggestion for interpreting it. You may use this or come up with your own interpretation of the Scene Type, literally or figuratively. Try to establish a scene with high potential for drama in any case.

Card:Scene Type
2:Division (The expedition splits into subgroups, temporarily or permanently.)
3:Abandonment (Someone is left behind.)
4:Dwindling Supplies (Food or Water are running short.)
5:Seduction (Love blooms among the sands, or shifting loyalties.)
6:Lost (The tools of navigation have failed: where are we?)
7:Mystery (One of the members of the expedition is not who they claim.)
8:Struggle Against Nature (Sandstorms, blistering heat, wild jackals.)
9:Power Struggle (Some are people want a change in leadership around here.)
10:Death (Will the deceased be mourned, or looked on as one less mouth to feed?)
J:Betrayal (A demonstration of the disastrous effects of misplaced trust.)
Q:Revelation (A long-kept secret is revealed.)
K:The Distant Past (Long before the expedition, what connected some of the members?)
A:Murder (What kind of justice can reign this far from civilization?)

Scenes should be played as freeform roleplaying. The narrator has 'ownership' of the results of events and to his own main character's actions (subject to being Edged Over, about which more shortly), but he should not overturn another player's description of the actions of their sub-character unless they make a substantial change in the direction he wants to take the scene. Players other than the narrator may inject their characters into the scene at any time, and can bring others into the scene as well if they wish. Neither the main character nor any of the sub-characters will ever die during a scene, but beyond that, anything goes.
Edging Over
All of the players in this game are what is known in literary terms as 'unreliable narrators'. Some may not even be human, and all are telling a tale that there is no living witness around to contradict them. As a player other than the narrator of the scene, if you have a card in your hand that has a higher rank (aces are always high in this game) than the scene card, you may "Edge Over", and question the truthfulness of what has just been narrated. When you do this, you, your are speaking in the voice of your main character, outside of the flashback and back in the storytelling session in the ruined city. The type of question you should ask is determined by the suit of the card you are playing, according to the table:
Suit:Question Type:Examples
Clubs:Outright Deception
"That's not what really happened, was it?"
"But it wasn't really a cat, right?"
Diamonds:Ulterior Motives
"What was your real agenda here?"
"Didn't you plan to get caught all along?"
Hearts:Hidden Feelings
"This disaster made you happy, didn't it?"
"You didn't let on, but seeing them deeply hurt you. Why?"
Spades:Deception by Omission
"What aren't you telling us?"
"But what did you do on the way to her tent?"

As you can see from the examples, you may as a general, abstract question or a highly specific one that suggests a single answer. After you have Edged Over, the narrator has two choices. First, he may accept your question and answer it. The answer should never be trivial; it should always drastically change the nature of the scene. Second, he may instead turn control briefly to the player who has Edged Over, answering "You tell me" or something similar. The player who Edged Over should then narrate his own answer, an alternate version of the most recent events in the narration. Once the question has been answered, he should return narrative power back to the original narrator. In either case, the card which has been used to Edge Over becomes the new Scene card for the remainder of the scene. Place this card on top of the old scene card.
If two players wish to Edge Over simultaneously, they should play rock-paper-scissors to determine which one may play the card. Playing cards brings you closer to your Epiphany and to the Endgame, so players should try to play cards whenever they can. However, each player may only play at most one card per scene.
Edging Under
The other way to play cards during a scene is by Edging Under. This is, however, considerably more difficult. Edging Under involves playing a card that is of lower rank than the current scene card underneath the card, which does not replace it as scene card. In order to Edge Under, you need to provoke or otherwise arrange for another character in the scene to do the specific thing to your character indicated on the chart based on the card you are trying to play. When attempting to Edge Under, however, you may not verbally suggest that the other character do this action, and you may not do the exact same thing to that character first either. The type of action involved here is determined by the suit of the card being played: clubs represent physical violence, diamonds represent dealmaking [The descriptions in the diamond list usually mention an offer being made. If you Edge Under based on such an offer, your sub-character must accept the offer made.], Spades are hurtful words, and Hearts are emotional responses.

2C
Poked with a finger
2D
Given something of value as charity
3C
Shoved
3D
Barter, exchanging two non-money objects
4C
Slapped in the face
4D
Be offered money for sexual favors
5C
Tripped or caused to fall down
5D
Give or receive a loan
6C
Restrained by grappling or wrestling
6D
Get an offer to buy one of your possessions
7C
Punched
7D
Get an offer to purchase one of another character's possessions
8C
Restrained with implements (rope, chain, cage , etc.)
8D
Receive a bribe
9C
Kicked when you're down
9D
Divide a windfall
10C
Assaulted with a weapon
10D
Get a better offer, a higher bid on something you've already agreed to buy or sell
JC
Deliberately tortured
JD
Be offered to buy or sell information
QC
Inflicted with a serious. life-threatening injury
QD
Be blackmailed
KC
Have someone attempt to kill you
KD
Be offered money in return for silence
2H
Be smiled or winked at
2S
Be told to shut up
3H
Be complemented
3S
Be called a fool
4H
Have someone make a pass at you.
4S
Have your morals insulted
5H
Have someone express concern for your wellbeing
5S
Be accused of lying
6H
Have someone comfort you
6S
Have your intelligence insulted
7H
Have someone demonstrate trust in you
7S
Be compared, unflatteringly, to something other than a human being
8H
Be forgiven for something
8S
Have your family insulted
9H
Have someone confess something to you
9S
Be blamed for a misfortune
10H
Have someone declare their love for you
10S
Be accused of holding a long-term grudge
JH
Be told a secret
JS
Be accused of keeping secrets
QH
Have someone trust you with their life
QS
Be accused to disloyalty
KH
Have someone beg forgiveness from you
KS
Be accused of a serious crime

Ending a Scene
Scenes should last approximately between 5 to 10 minutes. If every player has played a card in a scene, the narrator should attempt to wrap things up fairly quickly, and otherwise allow the scene to reach a natural conclusion based on a resolution of the central conflict suggested by the scene type. After the scene has ended, place all face cards and aces that were played during the scene onto the Tableau card. Each player who played a card during that scene should then draw a card from their pack. If that card is an Ace or a Face Card, they have an Epiphany. After all Epiphanies are complete, proceed to the next player clockwise.

Epiphanies
When you draw your Identity Card, you know who you are.
If your Identity Card is an Ace, you are a real human being. You are your main character. Your sub-characters may be past lives, alternate versions, or an extremely distorted account of yourself. You still need to learn who the other players are, and whether they can be trusted.
If your Identity Card is a King, you are a ghost. You are one of your sub character, and that person was murdered. You don't know who, but you do know that it was one of the other players. If you can get your murderer to trust you, you may yet have vengeance. If that fails, you may yet find peace if you can achieve mutual trust with someone other than your murderer. It would be best if everyone still believes you're a human being.
If your Identity Card is a Queen, you are an Angelic spirit. Your job is to protect the real humans here, which means getting them to trust you. If you are the one to end the game, you can reveal yourself to them, but if not, it's best for them to believe you're a human being.
If your Identity Card is a King, you are a Diabolic spirit. Your job is to lead the real humans astray, which means getting them to trust you. You should try to make everyone believe you're a real human.
When you have your Epiphany, put your Identity Card aside, face down, near your character sheet. Announce to all of the other players the suit, but not the rank, of your identity card, and place a marker on that rank of the Tableau. Shuffle your current hand into your pack and draw the top three cards from the stack of aces and face cards to be your new hand.

Endgame
The game ends when any player has no cards remaining in their hand. If any player has not yet had an Epiphany by the time this happens, they should immediately search their pack for their Identity Card. They should then announce the suit of their identity card and place their marker on the Tableau. They may then draw one card from the pack of Aces and Faces and look at it. The player who ends the game may, if their Identity Card is a Queen, reveal that card to all players. If their Identity Card is not a Queen and there are still undrawn cards in the stack of Aces and Faces, they may draw a card from that pack and look at it.
Then, each player writes on a piece of paper, keeping to themselves what they are writing. Angelic and Demonic Spirits should write "I am an Angelic Spirit" or "I am a Demonic Spirit", as appropriate. Ghosts should write "I am a ghost, and <character name> was my killer." Human beings should write "I am a human being, and I trust <list of character names>". All four sheets are then revealed simultaneously.
Identity
Scoring Method
Jack/Demonic Spirit
Score one point for each human who trusts you, then subtract the score of each human.
Queen/Angelic Spirit
Score two points for each human with a positive score, then subtract two for each human with a negative score.
King/Ghost
Score three points if your killer was human and trusted you. Score two points if a human who wasn't your killer trusted you. Then subtract one.
Ace/Human
Score one point for every human or Angelic spirit you trusted. Score one point for every Demonic Spirit you distrusted. Score one point for every Ghost you trusted who did not name you as their killer. Then subtract one point for every Demonic Spirit you trusted, every human or Angelic spirit you distrusted, and every Ghost who did not name you as their killer you distrusted. Finally, subtract two points if you trusted a ghost who named you as their killer.
Every player with a positive score can be said to have 'won' the game, although the player with the highest score may have more bragging rights. Players who were Ghosts are encouraged to narrate a brief mini-scene describing their death (remembering that it should be as one of their sub-characters in a real human's flashback.)