I really, really want to see co-op. There's almost always someone else in the room when I play games, and seeing games remain single player when they would be perfect for co-op, like KH, is a real pain. This would be especially true if it were on the Wii, as it's more of the social gaming console.
I have watched many Pixar films, so something from them would be great. I haven't seen Wall-E yet, unfortunately, but the setting looked amazing, so that should work. I don't care what other Disney worlds are in, as long as they're colorful or look good. Going through that Tron world in KH2 was a damn nightmare... Oh, and Mulan owns, a reappearance would be welcome.
As for the system placement, I'd prefer the Wii. The system isn't getting nearly enough (or any) Square Enix love, and I think it would promote the idea of adding co-op into the game more. The series isn't in need of an HD graphics boost, imo, so it'll be fine - that should also cut the development time down a good amount. It shouldn't need the motion plus either, as that sounds unnecessary for an ARPG. A combat system more like NMH with RPG elements would be great.
Oh, and someone mentioned duo / triple techs; that would be bad ass. Put em in!
