OK, so I should probably start this Game Idea by citing its influences: Sly Raccoon (PS2 series), Time Bandits (movie), Suikoden, Kingdom Heart's World Map system, Gain Ground (several 8- and 16-bit consoles) and an old D20 game system called "Tales of the Floating Vagabond".
The premise is thus: You control a legendary team of highly effective bandits - taken from various historical eras and fictional worlds - which your father put together after discovering the Nexus. The Nexus is a floating mass of land surrounded by interdimensional portals to practically every other realm of reality that could possibly exist, including eras in the past and worlds which have been imagined; providing them with a basis to exist. Your father has long since retired with his earnings, taking most of his team with him to an unknown "retirement dimension". He has left you his foreboding base of operations (the Skull Citadel) in the Nexus with which to carry on the legacy of his thief gang. He also left a few portal destinations on the Citadel's all-powerful computer system Omega.
So that's the outlandish premise, so how does it play? Well, since I'm sort of skirting around Kingdom Hearts and Sly Raccoon territory, I'll downplay any platformer element to it. It doesn't necessarily require an RPG element either, but I like those so we'll include some. I'm currently thinking of presenting it as an old-school top down isometric-based game, as those were always my favorite when playing a game that required sneaking around. The various angles and vectors used in isometry makes it perfect for judging lines of vision and the effective range of hiding in shadows and the like. It's a math thing, and one that's very important when it comes to the stealthier side of gaming. I tend to find with FPS and Platformer Stealth games that it is much harder to judge those kinds of variables with the POV you're given.
Obviously, if I am to use this sort of graphical system the game will be regressing somewhat to a simpler age, which will make it more ideal for a handheld like the DS or PSP. But as the excellent Contact (DS) has shown, games using this almost obsolete 2D system can still look and play beautifully even compared to more modern fare.
Now, I'm not saying the game will be strictly stealth. There will need to be a combat option for instances where the player might not want to steal stuff by sneaking around all the time, and the system that makes the most sense is a turn-based system based on Fallout's GURPS or the standard D20 ruleset. Because of the sheer scale of the project (I plan to make the amount of time eras you can visit immense in number, to make up for the shortcomings of the 'classic' graphical style) there will need to be a strong (and therefore existing, to save on way too much testing and tweaking) set of rules governing things like lasers vs longbows. Each stage will have a clearly defined goal (some legendary treasure of that area) with several ways to make intermediate funds in the process, rewarding players who take on extra challenges and explore thoroughly which tends to be something pretty much all my games tend to reward. Sort of a burgeoning eccentricity of mine, really.
Now, onto the actual artistic style of the game. I was somewhat inspired by what Kingdom Hearts 2 did with the Timeless River stage, set in the world of early Walt Disney cartoons complete with black and white 30s renditions of the main characters and scratchy music. It would be awesome to do something like that on a grander scale, allowing each time era or world to have its own artistic style and drop the often alien components of your bandit team in there for their latest caper.
I'll lay out a couple of examples of how a player would find a stage, take it on and how it would play out:
Egyptian Stage: You'd receive word from your supercomputer Omega (which may need a better name) that a gate has been stabilized to Ancient Egypt in the "real world" section of the Nexus. You would assemble your team, perhaps unknowingly leaving behind a computer expert and laser grid disarmer (most of your units will have their own, albeit not unique, specialities) and instead assemble a few warriors to back up the infiltrators, the hero character and perhaps an acrobatic thief chick you picked up sometime in the modern era to avoid the booby traps of the Pyramid you're about to rob.
As soon as you get there, you discover that this era of time had actually been taken over by aliens (sort of a nod to Stargate) and they have all the same laser-grid and computer technologies that you were hoping to avoid dealing with. You can choose to escape to retool or continue with the team you have. Using the isometric view of the nearby desert, you decide to fight your way to the pyramid (since you can't easily hide in open desert) and allow your two thief character inside while the warriors stand guard. Between the two of you manage to find the artefact you're after and regroup with the warriors outside, making a mad dash to the Nexus before the aliens use their full force against you (which would be a UFO with a particularly lethal looking death ray).
Alternately, there would also be a few side-quests available to you based on whether or not you want to spend that extra effort (or are particularly skillful at this point). For this stage, you could find a way to infiltrate the UFO from the Pyramid and take it out, saving the denizens of Ancient Egypt from becoming slaves and perhaps recruiting one of the temple guard NPCs you befriended there to assist you as new muscle.
Film Noir Stage: A hyper-realised version of a Prohibition-Era US, inspired by many film noir movies and books. The artistic style will mean everything except your team will be in greyscale, making them more obvious looking. It may be possible to have greyscale characters from other areas too, or something like a Ninja which would dressed almost entirely in black, either of which would benefit from being in a greyscale world when it comes to interacting with people or blending in.
The target treasure in this case will be in a guarded Mob Boss location, which means a preference for characters with firearm experience and possibly an explosive expert with which to storm the place (you don't need to worry about body count since this is a non-historical area and the mobsters are all violent criminals). Side-Quests could include smuggling alcohol from other dimensions into the Noir universe (which could get you all sorts of interesting goods), robbing rich citizens (which would have an effect on your morality score) or assisting a mobster assassin to take his girlfriend to safety (through the Nexus) so the Mob boss won't be able to use her to blackmail him into killing rival mobsters: thereby acquiring both him and the girl.
As a final design note I'll describe a feature the game will use that monitors two aspects that define your personal method of completing heists:
1) Morality or "Honor Among Thieves": This will effect character progression and hiring new people with different ethical perspectives (i.e. good guys and bad guys), sort of like the ubiquitous Dark Side/Light Side factor in many RPGs similar to this game.
2) Time Elasticity: This is a little more out-there, and will be hard to implement, but the game would be so much cooler with it. It's basically the idea that if you bend time a little (to steal some insignificant artefact, say) it'll snap back and there won't be any major changes to the future. Change it too much however, and the timeline might be irrepairably changed. This would only effect stages based in the past, but if you were to break the time elasticity by doing too many things out-of-character for that era or killing too many people you would fail the mission by causing a time paradox and destroying the Nexus: It's pretty fragile since it has portals to universes which need to stay constant, and losing the Nexus means pretty much losing your entire base, your crew, your computer and stranding you in whatever time era you're in. Which would be bad.
