Well, let's focus ideas on what's already mutually popular around here:
-Versus seems to be pretty keen. Multiplayer in general, in fact. And writing good AI (unless somebody here is really good at that) can be a PitA; so I'd recommend that PvP should be the rough equivalent of boss encounters (and a large focus of the gameplay).
-Procedurally Generated Content is of high interest, along with crisp level design... so if we can "procedurally generate" well-designed levels... just take a level design with "open entry" and a limited exit (as well as a "starting point" that force-feeds you through one exit, into the opening level), and stack them together. If scrolling, a sublevel's design and flow could be entirely changed this way, simply by the previous level's exit point acting as an alternative starting point in the new level. Narrative flow could be created somewhat via exit points, as well.
-Demon's Souls is a major hit here on a number of levels - it's "friends or foes" structure, difficulty curve that assumes you aren't a noob to gaming, ability to invade other worlds, forum-like messaging structure, and how picking different characters or classes provides an entirely new strategic experience. I have a number of ideas of how to implement this kind of stuff; but that depends entirely on what creative direction we intend to go with this project. So I'll discuss that later, if this thing does take off.
-Shmups seem to be blasting off lately here as well; and they're versatile in how they can be developed - in fact, it's one of the few genres that works well as either side-scroller, vertical-scroller, OR open-movement. Racing games can do that to, to a certain extent; and there's really no good reason a shmup and racing game style couldn't coincide, at least somewhat. In fact, I think more influence from one another would only make both types of games more fun. And a vehicular gameplay focus would probably result in easier/less spritework, as well.
-MvC 2/3 is an attention-getter too. Perhaps incorporating ideas of special moves from that into attacks could be fun. Ever see a
ship combatable vehicle* pull a "Molecular Shield," "Psy-Thrust," or "Proton Cannon?" Would you ever like to?
*Why limit thinking to "flyers?"
-Many of us love platformers and Metroidvanias, too. I could see a "Metroidvaniaesque" advantage system coming into play somewhat - like winning a certain event/objective will give you an ability to more easily traverse terrain or a combat advantage. Incorporating platformer stuff... might be easier as a side-scroller instead, but again, that part's debatable. But for sake of simplicity, I think we'd all opt to not do a mixed setting (think NES TMNT).
Originally, I threw this idea out in the Versus compo. It was about a resource-managing co-op/competitive RPG with a roguelike setting and RPG'esque PvP battle system. But with a little rethinking; we could take the basic outline of that, and change it into a co-op/competitive "adventure/racing/shmup" game with intrinsic hazards beyond just that of other players. And perhaps even incorporate your user-avatar here as your "character" window; so people know just who you are, here.
This could also provide RPG-like elements like status effects and elementals in a non-RPG-typecase kind of way. And while yes, a shmup/adventure at it's core, I'd also recommend avoiding an outer-space kind of setting, as well - that's usually a killer in the level design department.
Some more character/NPC influence is pretty obvious:
- "I control the gaming world, woohaha!" and of course, the bespectacled-joystick logo.
*EDIT: Divided wall of text for readability.
*RE-EDIT: Upon further thinking... a psuedo, forward-scrolling 3D kind of game could do all of the above, AND incorporate the platformer elements, as well. Think "if Sonic 2 worked like Crash Bandicoot," somewhat. But again, all totally up to debate, regarding the creative direction and further input.