I don't know if anyone's said it yet (ie haven't checked the whole thread), but...
JC2 and Mass Effect
Granted, I've only played the demo of JC2 and have never played Mass Effect (don't have a 360, computer is old), but picture this; the awesome sandbox nature of JC2 with all its planes, hook-things and destruction, in a Mass Effect universe. You'd be able to surf space-ships between planets, hook two suns together and watch the mayhem!
That and the game'll actually have a good story. You could even chuck in some RPG-lite elements from Mass Effect into it.