Ideas for anything goes setting

[Stormlance]

This is something that has been knocking around in my head for a long time now. I want to build a world/setting in which just about anything is possible. I envision a semi post apocalyptic setting with desert and rocky wastelands, with small pockets of more verdant terrain scattered around. In this setting there would be advanced technology from the pre-apocalypse world which is patched together, as well as an abundance of the typical post apocalypse gear. Magic would exist though I'm not sure if I would want it to be D&D levels of sorcery or a more limited, ritual based system.

To offer multiple options for players, there would of course be regular humans but also mutants, aliens, a few Demi humans, possibly robots, and just for fun an option for time displaced people from different eras of history or are "lost" in time.

This is what has been kicking around in my head for a while and I'm thinking of starting to work on it. I haven't decided on a system yet though. I'm leaning toward d20 Modern as it already has a lot of what I'm looking to add, or possibly True20 which I like but haven't had much chance to experiment with it.

Anyway. Thoughts? Suggestions?
I'm just spitballing here, getting ideas out to help make things clear for myself and get the creative process flowing. I'll post more as things develop and maybe turn this into a project thread.

[Yora]

When everything is possible, nothing is surprising and planning impossible. It also almost always means that of everything there is, there is only very little.
Are you sure you want that?

[Grinner]

To offer multiple options for players, there would of course be regular humans but also mutants, aliens, a few Demi humans, possibly robots, and just for fun an option for time displaced people from different eras of history or are "lost" in time.

As I tend to like kitchen sink sort-of settings conceptually, I won't try to dissuade you from this entirely. That said, I will point out that you're already shooting yourself in the foot. For a world to work, it should be cohesive; if you drop UFOs into your medieval fantasy, there had best be a good reason.

So...Where are these time-travellers coming from, why are they there, why can't they just leave, and what effect does their presence have on the world at large socially, economically, and cosmologically?

[Stormlance]

Again let me emphasize that these are just random raw ideas bouncing around in my head right now. I'm not 100% sure what will be included in the setting and what wont.

Right now I'm working to solidify a theme and genre for the setting.

[Grinner]

Right now I'm working to solidify a theme and genre for the setting.

You've pretty much just established that, right? Something like Gamma World?

[Stormlance]

I'm not familiar with gamma world

[Murk]

In response to what the other posters have said, I think this "let's throw everything together"-idea can work, and it sounds exciting enough, but don't try to play it as a stable situation. Mediëval Knights, cowboys and aliens together can have some good plots and frictions, but let's be honest, before fifty years have passed the knights have learned to shoot lasers and the entire world will turn full-technic.
So, either you have to come up with convoluted ways of keeping the cultures isolated - which dissolves a lot of the excitement - or you'll have to accept that these campaigns will take place in the very small few years in which everything is new and unknown. The time travellers have arrived last year, the aliens haven't been around much longer, the apocalypse happened ten years ago, and magic is still pretty unknown.

Having said that, this setting offers so many interesting possibilities. I'd love to see a war between robots and undead, both claiming they are "real individuals" and the other just "mindless constructs".

[Everyl]

Have you ever seen the Feng Shui RPG? It's been years and years since I've played or read it, but IIRC it covers a great deal of what you've described, so you might be able to draw some ideas from it. In particular, it has a sort of "clash of the eras" vibe that might be useful for working around the issues some have suggested with time travelers. IIRC, a big part of that was that the magic in the setting was mostly/only known by people from more ancient time periods, who lacked the training/knowledge to use modern/future tech.