As a bit of a change of pace from all these mechanics-first suggestions, I've got a concept that I'm quite far from implementing mechanically:

The character's experience is a posthuman initiation rite designed to prepare humans who have recently died for the transformation to digital life.

In style it's like a mix between the egyptian book of the dead and celtic stories about entering the world of the faerie (the interface between the two is roughly about the investigation/purification of the last life and the initiation into the next).

The big tricky thing is I'm trying to work out mechanically is a journey that is thematically appropriate to those who walk it, similar to how Vincent encourages the GM to push on people's judgements in Dogs, I want each to further push on the identity of the characters.

But what's the objective? What are they pushing the players towards? Well firstly I want it to multi-final, so it's not about funnelling them to the same personality, but equally, I don't want them just to be subverting whatever they say about their character, more forcing them to come into a different relationship with the world. I think I'm going for a pantheon of GMs (probably three) each with a slightly inhuman but different focus. Ideally I'd like to start mythic and build similarities with computational stuff as they occur, so I'm not limited by modern technology.

So that's the challenge, how to set up the scenes to impinge on the players concept of their character, and force it to come to terms with the new realities represented by the three GMs, generating stages of the journey from the changes in the character.

Perhaps I've been out of the loop too long, but you aren't making much sense to me. Need to get my English-to-Forge handbook out again...

I assume the 'journey' from human-epistemic life to posthuman-digital/mythic life is effectively character creation (as opposed to the meat of the game itself)? Or have I got that wrong and the entirety of the game is about the developing player characters through various 'allegorical' challenges? 

If it's the latter perhaps you should initially task the players with inventing flawed characters to undergo the 'journey'. The body of play is the confrontation of these flaws as manifested into a mythical world.
So a character might be represented like so:

In life I was...
"A compulsive liar"
"A glutton"
"A gambling addict"
(or something)

So as the player-character travels through the mythic world he or she might be set upon by Sphinx demanding the truth, tempted by a deadly fae feast and challenged in a spirit's gambling-house. These confrontations would effectively be key-scenes where a characters flaws are presented to the group and discussed/resolved. If the challenge-scene is overcome the player gets to change their 'trait' to a positive one that might be used to help fellow travellers, if failed their flaw becomes worse ("A compulsive liar" might become "I cannot tell the truth") that may impact on the rest of the party.

So play, while aesthetically being a journey through a mystical/mythical land, is effectively about continual character growth/change and its impacts on both the individual and the player-party. 

Apologies if this is a crazy digression.

M. wrote:
If it's the latter perhaps you should initially task the players with inventing flawed characters to undergo the 'journey'. The body of play is the confrontation of these flaws as manifested into a mythical world.

While it's important to retain a sense of sympathetic normalcy in the player-characters (the golden rule of protagonists is either 'ordinary people in extraordinary circumstance' or 'extraordinary people in ordinary circumstance') you should avoid allowing them to become dull. Allow players to throw curveballs if it makes for interesting, effective and memorable characters! You'd be surprised how many "normal" folks are addicted to drugs/gambling/sex, lie, steal, cheat, abuse their positions and commit adultery, theft and murder.

If we consider the cast of Lost, to use your example, we can see that, while each is effectively a normal human being, their inherent character flaws become more and more problematic as more and more pressure is applied. Even a mild selfishness or understandable cowardice can be transformed into a crucial character-defining trait when lives are on the line; do you save the doctor or is it every man for himself?

I think that challenges presented to the players should test their characters as 'normal' people: do they surpass their own flaws and become better people for it? Do I master my fear of the Ju-Ju Monster and go back to save Steve dangling on the edge of the precipice!?

Hollywood films do this all the time, especially family action-adventure films. Robin Williams' character in Jumaji overcomes his flaw of "I fear the Jungle" when what's precious to him is at stake: he wrestles a crocodile when his character's love-interest is in jeopardy. Challenges need to test a characters flaws, not strengths (à la D&D..), when something important to them is on the line. It makes for entertaining stories and, as any scriptwriter will tell you, efficient character growth. Of course, this can be subverted easily to make stories darker, tragic or simply more complex. The plot of Hamlet consists almost entirely of failing to overcome character flaws.

Characters both need flaws to be challenged and a precious thing(s) to be at stake (in traditional order of importance: their wealth, their happiness, their life, their friends, their family, their love-interest, their nation, their world, their universe).

As for the aesthetics, I leave that to you. You can either play with the players as a travelling band/party or Polaris round-robin style (but never let one player have a whole session hogging the spotlight!) - I think either is doable with what you're describing. A party is a double-edged sword in that while they can work together to tackle challenges, they can also become each other's challenges through character-conflict - both being good for your purpose. Round-robin will work - but what will it add?

So you are trying to figure out how the course of the game might go, and then you’ll worry about figuring out a mechanic for that?

Perhaps a journal of “self-discovery” might be a good framework. When a person “dies” they enter this afterlife-like place with a lot of preconceived notions that might not be actually correct. They think they are so honorable and virtuous. Conflicts arise (such as the aforementioned Sphinx) and in the course of those conflicts the characters discover that they aren’t as virtuous as they thought. Maybe they lie to the Sphinx and now have to deal with the realization that they aren’t an honest person. A few more conflicts and maybe they discover that they are a in fact a horrible person. Once a virtue hits a certain level of “vice” that vice might manifest in this world and the player(s) must go and directly combat it (or perhaps they are already on the way to deal with their own vice and the conflicts along the way either strengthen or weaken it).

If you want to include more than one player in the action at a time, each player might represent a perceived virtue of the person who is passing on (manifesting in the world as a separate character, but the destiny of each being interrelated). In the case of the truth, the virtue of “Equality” might save Truth from being degraded to a Liar by taking the hit instead. The person *is* honest… but they are only honest with men (thus “Equality” begins to take the slide to “Sexist”).

Once each “virtue” has been purified, the person as a whole can enter the next world. Or if the Vices defeat the virtues, the person gets sucked into a repository of icky souls.

This reminds me a bit of After Life, the film by Kore-eda Hirokazu.  If that were the inspiration for a game, the deceased characters would relive various scenes from their past, before selecting one to take with them into the next world.  In their capacity as directors, stagehands, and extras, the staff of the waystation take on many of the functions of a traditional gamemaster.  In some ways, this is similar to your posthuman conversion.  Perhaps the dead are trying to reconstruct their memories and select one that will define their new existence.  That sounds a bit like A Penny For My Thoughts too, but I've never actually played that game, so I could be wrong.

In your game, does each player control a dead character?  What if these characters are all working together to help each other reproduce their memories, rather than being locked into their own private bardo states?  In that system, there would be no separate pantheon of gamemasters -- the characters themselves would fill that role.  Every player has certain assumptions about his or her character, but because other players are directing and acting out their memories, those assumptions will be challenged routinely.

For example, Omar, Sarah are two of the characters.  Their souls were transferred into a simulator for processing.  Before they can advance to the next stage of their existence, they must help each other to reproduce key memories from their lives.  When Sarah acts out a scene with her husband, the other characters are the one actually manipulating the simulator to produce it.  Sarah doesn't control her husband in that scene, but she gets to choose which character will act out his role.  She chooses Omar, because he most reminds her of her husband.  She also choose which character will direct the scene as a whole -- that person doesn't act out any roles, but instead acts as a producer, with authority over all minor characters, random events, etc.

At the end of the game, each character chooses one of their scenes to take with them -- this is what most defines them as a person.  If Sarah picks the scene that Omar directed, then Omar gets some kind of reward.  Perhaps Omar is only allowed to advance to the next stage if someone else chooses one of the scenes that he directed.

Thanks for the responses guys it's giving me a lot to think about:

Basically I'm going for a pretty unusual narrative form here, and I'm trying to work out how much I should pick up these good ideas and how much they would dilute the specific vibe I'm going for.

M. wrote:
I think that challenges presented to the players should test their characters as 'normal' people: do they surpass their own flaws and become better people for it? Do I master my fear of the Ju-Ju Monster and go back to save Steve dangling on the edge of the precipice!?

J wrote:
If you want to include more than one player in the action at a time, each player might represent a perceived virtue of the person who is passing on (manifesting in the world as a separate character, but the destiny of each being interrelated). In the case of the truth, the virtue of “Equality” might save Truth from being degraded to a Liar by taking the hit instead. The person *is* honest… but they are only honest with men (thus “Equality” begins to take the slide to “Sexist”).

Once each “virtue” has been purified, the person as a whole can enter the next world. Or if the Vices defeat the virtues, the person gets sucked into a repository of icky souls.