I am entirely inexperienced in the realm of game design, but what the hell, I have hubris enough to believe I have a shot at making some horrible, mottled monstrosity which people might play out of pity for its limping gait and drooling mouth. So here goes nothing.

Yeah.

Sojourn is a city which crawls across the land, leaving desert in its wake. It consumes the resources that it finds along the way, though for what purpose, only it knows. Those who are unfortunate enough to have been drawn into the city eventually lose themselves to it, and become Denizens, men and women with Brass and clockwork stitched into their skins. Engineers, repairmen, laborers. Worker bees in the hive that is Sojourn.

The characters all crave something so strongly, but it is something dark, something forbidden, which they could never obtain on their own, within their home societies. Each character will likely crave something different, but each also hears the call of Sojourn. Sojourn promises anything they could desire, if only they will come and serve it for a while. They have heard the call, and they could not resist. Now, they make their homes inside its tarnished depths, and feel it worming its way into their heads.

A choice is presented to each of them. Serve Sojourn, assist it in its mission, or fight against it and sabotage its efforts.

Sojourn seeks to consume resources to accomplish its goal, but many of these resources are difficult to obtain, for whatever reason. Perhaps because a forest tribe defends the trees, or because some creature lives in the cave full of diamonds. Whatever the case, Sojourn requires the assistance of those it can send out, to go and weaken these threats, that it might obtain the resources it needs. The characters are thus sent forth with explicit missions to pave the way for Sojourn's coming. But any of them can choose to stand against it, instead causing trouble for Sojourn, trying to slow it down.

After every such endeavor, the character is granted either more Brass, making him or her ever more a denizen of the city, or he or she learns more about Scars.

The people outside of Sojourn mark their saviors and heroes with Scars, sigils carved into flesh in order to hold back the dark influence of the Eating City. If the characters are willing to accept these Scars into their skin, then they may gain enough power to truly resist Sojourn and fight back. But it will come at the cost of whatever dark thing those characters desired from Sojourn in the first place, as there is no way that they will ever be able to obtain their darkest wish without Sojourn's help.

Ultimately, if any of the characters do break free of Sojourn, then it will come down to a final conflict, to see if Sojourn can fully be defeated and prevented from achieving its goal, or to see if it will consume all it needs with impunity. The characters who would serve Sojourn and get what they crave stand with it against the characters who sabotaged its efforts and tried to stop it. Only one side can ultimately prevail.

So, after that melodramatic description...

Rules ideas I've had so far include having one player play Sojourn itself. This player is the GM-ish player. He or she will choose locations on a map made at the beginning of the game for Sojourn to travel to and consume. Each of these locations will have been assigned a certain number of resources during map creation. The Sojourn player will roll for a random goal on a table, and this goal will tell him or her what resources and how many he or she may need to accomplish his or her goal. So the Sojourn player will likely choose places to consume based on this information.

At every location, the Sojourn player can give the other players a mission or missions to help it consume. Ultimately, Sojourn will always consume the location, and will turn it into an empty desert wasteland. But having the help of players from successful missions will give it a boost in its efficiency roll, which it makes every time it consumes a location. The efficiency roll determines exactly how many resources Sojourn gets successfully.

Meanwhile, the players all have a couple of important elements. They have their dark desire, which only Sojourn can grant them. They have Morality points, which indicate their willingness to pursue their dark desire. And they have a Skin stat, which is split into Brass and Scars. Brass + Scars = Skin. So, if you have more Brass, you have fewer Scars. And of course, every point must be defined, so that a point of Brass must be described as something which Sojourn has put into your skin, like a spigot or an exhaust vent. When on a mission, Brass will determine how many dice you roll on any given instance, while Scars will determine how many rolls you can actually make. This is because all the characters are mentally drawn to Sojourn, and if they leave for an extended period of time, it begins to burn away at their sanity. So you have a limited number of rolls you can make before you have to return. I'm also thinking about providing players the opportunity to make more rolls, but at some kind of cost. Maybe Morality points, which I was envisioning as a resource not unlike FATE points, to be used when pursuing a goal related to your dark desire. Morality points would, in the final conflict of the game, with the rebels against Sojourn, add to the dice pool available, which is the primary reason why a player might choose to conserve them instead of spending them.

Every time a mission roll comes up, a player has a choice of rolling either for or against the Mission. If he or she rolls against the mission, then every success counts as a die which might subtract from the efficiency roll for that location. If he or she rolls for the mission, then he or she rolls against a difficulty determined by the Sojourn player. Higher difficulties give greater rewards for the efficiency roll. If a player succeeds in a roll in favor of a mission, then he or she can gain a desire point. At certain intervals, the player may spend desire points in order to obtain elements of his or her desire. Ultimately, the players who are choosing to aim for their desires must try to earn a certain number of desire points, before Sojourn gets enough resources to accomplish its goal, and the characters who remain on Sojourn become Denizens.

Now, obviously, there are a lot of little mechanics running around here. I still need to work out exactly what the dice mechanic will be, and I need to work out the ending conflict mechanics. But I figured I'd outline some of my goals here, just so that they're clear.

(1) I want the GM to have his own conflicts and game elements which require him to employ strategy, instead of just having utter power over NPCs and stuff.
(2) I want the players to have to make a choice between siding with Sojourn or against Sojourn, based on a number of different factors.
(3) I want there to be some kind of final conflict between the two sides, which is built up to over time.

I feel like there are elements which I thought about which I haven't included here. If they occur, I'll put them up later. Any comments would be greatly appreciated!

I like this... it seems almost like a roleplaying board game in which the board game element (the city of Sojourn moving around and eating shit) acts as a framework for the RP (determines the scenes) which then really heavily influences the efficiency of the city.

How do the players defeat the city? Why can't they stop it from burning through a location?
How about PvP, considering individual characters actually pick sides... this seems like a real possibility.
What incentives does the game give me, as a player controlling a character, to work against Sojourn?

Also, have you thought about the tone of the game... I could see this being really dark ala Dark City, savage ala Dark Sun, or something more in line with a Miyazaki movie (Nausicaa, but even lighter, like Spirited Away) and I'm sure, depending on how you present it, you could come up with a bunch of other ways to present this setting.

Yeah, the board game thing is definitely one of the overriding motifs I'm interested in here.

As for why the players can't stop Sojourn from killing a section, that's because it's the ultimate conflict of the game. When the final conflict comes, that will determine if they actually manage to stop Sojourn or not. Until then, though, they're along for the ride. If they really want to actually stop Sojourn, then they have to engage in that final conflict. That's the only time when the rolling deserts can be halted.

This game is sort of turning out PvP, but it's turning out to be an odd form of PvP, in which the players get to pick which side they're on. Sojourn is always one side, and theoretically, every player might choose to help it, making the story one of tragic consumption and submission to one's desires. I like the idea that you get to pick which side you're on, though, based on a choice of right and wrong and desire.

Incentives to work against Sojourn...well, in a way, I'm envisioning classic player mentality functioning to make this option more desirable. Although arguably, this mentality exists less in story games. But I was sort of envisioning that players might naturally want to resist Sojourn, just because they don't want to be controlled, don't want to be servants. And that is part of the incentive, too, as if you work with Sojourn, you may get your dark desire, but you'll also ultimately become a Denizen, a mere cog in Sojourn, and nothing more. To become an individual, a hero, you have to stand against Sojourn. Also, to stop Sojourn from consuming all and killing the towns and villages from which the characters originate, they must stand against Sojourn. So if they're willing to let Sojourn's Desert consume everything they know, just so they can fulfill a selfish desire, then that's fine. But if they at all want to stop it, then they must stand against Sojourn.

As for tone...I will admit that though I have heard great things, and though I have very much intended to educate myself, I have not seen many of Miyazaki's works. I think I've seen Princess Mononoke (assuming I'm not being stupid, and it is a Miyazaki), but that's it. I did think seeing Howl's Castle would be appropriate, as Sojourn has something in common with that idea. But I think I'm moving for a much harsher, crueler tone. Sojourn is consuming a world which, until Sojourn came, was something of a natural paradise...but it was also filled with people with absurdly harsh taboos and codes of conduct. There is no right answer here. There are just choices, and what people are willing to sacrifice to pursue those choices.

To elaborate on what I mean about the cultures, I had a few thoughts about the world in which you'd play. You still get to create the map, and you get to design your cultures, but one of the overriding concepts will be that of the taboo. Every player gets to pick one taboo which is held by the people of the world outside of Sojourn, and then every player must choose another player's taboo as the basis for his or her own dark desire. So you might choose the taboo, "No love between people of the same sex," and then the dark desire of my character might be "To be with Tarrn," in violation of that taboo. Part of the point here will be that the characters' "dark desires" are really only going to be dark in light of their own cultures, per se. There still might be taboos we can support, and dark desires which do feel wrong, but ultimately it's all couched in terms of the cultures created for the game. Sojourn, of course, is the city in which there are no taboos. Also, Sojourn is the only city. Everywhere else is just small villages and towns, at the most.

Furthermore, I think I want for all cultures to, of necessity, have a taboo that no person should show any more skin than he or she absolutely has to. Except, the characters, with their Skin scores, and their Brass and Scars, have to show skin. Brass might make it literally impossible for them to wear clothing without showing skin, and Scars would be meaningless, perhaps even sinful, if not displayed outwardly. So these characters are inherently violating this taboo of skin, and becoming outsiders. Even the Scarred characters, who are being marked as saviors and heroes, are also being marked as outsiders.

I've had a few other ideas, too, which I will elaborate in another post, just to try to break things up a bit.

Hmm, if I were in your position I might consider leaving Sojourn as a more fixed and deterministic character.  Instead have the GM with his own character sheet and resources of some kind that he may allocate on the side of Sojourn's minions, and on the antagonists.  The more power that Sojourn and crew have, the more power the resistance does.  I think that would allow for dealing with concern #1 in your original post. 

Very interesting, if a tad daunting.  You wrote that all up today?  Wow.

Hmmm. The idea you propose, Jason, is certainly interesting, and it's gotten me thinking about having two GMs, one playing Sojourn, the other playing the People, and the Players existing on the *cough* EDGE between them. The PCs would have to pick which side they'd go to by the end of the story, and they would wind up being the most critical factors in favor of either particular side, the grains of rice which would tip the balance. I'm hesitant to make it out and out adversarial, because I didn't want the game to of necessity become Sojourn versus the Players. I wanted the players to have to make the choice as to whether or not they would join Sojourn, so maybe by including a representational element for both sides involved, it would make them clear options. Also, it would decrease the number of main characters present in the story, making those characters all the more important.

Yeah, I wrote all that up Friday night and Saturday morning. It's sort of the benefit, and the price, of being a windbag. Or, in terms of writing on paper, an inkbag. Which makes me a bytebag on the Internet?

I am curious, though, about exactly what you meant by daunting, you'd be so kind as to elaborate. Do you think that this entire thing I'm describing feels to monumental to really get into and play easily? I'm curious if it's sounding too much, even now in these preliminary stages, because if so, then I'd probably do well to trim back.

I suppose that my comment "daunting" was due to the richness of the setting and the general scope of the story.  A number of the games presented on the forge thus far for Game Chef are tailored to convention play and are by design, minimalistic.  By contrast, your game as presented seems much more expansive and I personally am not as ambitious with my Game Chef design. 

That being said, it sounds perfectly doable.  You will simply need to take advantage of your way with words to describe the world.