@OP
If you're just starting from scratch, two things that I think are consistently lacking are these. A game with really rich social mechanics that are well integrated. That can be almost any setting, but you pretty much never see that as a real feature of a game system. And, a game that is truly mythic in a lot of interesting ways. The thing that attracts me to concepts like fey or gods or whatever is that they have a core concept or bailiwick or nature. They are bound by that and it helps define them. Being the Dweller at the Crossroads or the Queen of Air and Darkness tells you important things about those entities. But, even if you go to the White Wolf games that purport to do some of this, you kind of get nothing really that works and helps you develop what I'll call a personal mythology (for lack of a better term).
Other than that... ideas that hopefully aren't too niche...- Mad Max: Fury Road shameless ripoff. The satisfying car chase mini-game would probably be the core mechanic. Still, it'd be bad ass.
I actually ran this in Savage Worlds, but adding a supernatural element as well. Figure pretty much Mad Max meets Deadlands. That was my campaign, and I was pretty proud of it. Savage has pretty good vehicle rules, they're lite enough to wrap your head around at least, and we had a pretty good time.
- Borderlands shameless ripoff. I've thought about doing this several times. I love the IP and the aesthetics. I'm not sure if you could get the aesthetics into the game, or not. This is actually pretty close to the Mad Max: Fury Road ripoff game, but minus the solid driving mini-game, and with psionic girls.
I've always thought ORE, probably in its Wild Talents incarnation which is the version I know best, would work well for Borderlands. Savage would work ok, but BL characters can do more superhuman crazy things. Savage doesn't have quite the mechanics to allow a player to invent mechanics for their insane powers and guns. It's really hard to build the beautiful awe-inspiring lethality of, say, Krieg in Savage.
Wild Talents, as an effects-based system, does. And, while I love M&M as a system, I'd want something grittier and faster than it. Wild Talents has nice initiative and hit location rules that I think would simulate fast-paced gun battles.
D&d has always been where I get my fantasy fix, but I've never found a sci fi game that I can really get into.
...
Scifi games do seem to uniformly suck. And, for the life of me I'm not sure why. I think there's a setting problem. Fantasy is relatively well-defined, and while D&D has a
very idiosyncratic take on it, we're all so familiar with it. Scifi runs the gamut from Blade Runner to Lensman. So, scifi games tend to be either appallingly generic, or have 1000 pages of setting material. And, they do just generally seem to really suck.
For scifi games, I tend to look at IP and source material that inspires me. That helps limit the vastness of the genre. I would
kill for a great Warhammer 40k game (I find FFG's almost uniformly terrible). I would also love a solid Dune-esque one (Fading Suns came out ages ago, and I think was kind of along those lines, but I don't remember loving it).
I was writing on a sci-fi game about aliens that got stranded on another world. Each player would be able create their own alien race using a point buy system. I kicked it sound for a while, but never really got anywhere without help. The goal of the game was to either find a way to escape the planet or to set up long term survival on the planet.
Not to pimp Savage Worlds, and I'm not in love with the system although it does do a few things pretty well, but it sounds like it might be a decent fit for what you're aiming at. Although I'd be worried that various subsystems like food and settlement building would bog down the game and make it feel like a boring tabletop version of Civilization. At least that was my experience when someone did a kind of post-apocalyptic version of this.[/list]