Remember Sonic vs. Mario at the Olympic Games? Talk about disappointing. You finally get both characters in 1 game, and it's a freaking Olympic game and not a platformer at all!
Personally, being the comic nut that I am, I'd like to see a Marvel/DC crossover. Thing is, if that ever did happen, it would probably be a 1-on-1 or tag-team fighting game. I'm just not into fighting games. I'd want to see this as an action-RPG like Marvel: Ultimate Alliance. That would be sick.