I can see totally see a Tekken character being added since Namco bandi made the game. That being said.... maybe a Gundam character, but that would be a nightmare in rights.
If Capcom had their way they would probably add Chris, but what is a notable resident evil song. I can see fans pushing Phoenix Wright, but what would a Phoenix Wright stage even look like. I guess they can fight in a court room. or go even meta and dodge yhe text on the bottom and the touch capabilities in a new game.
Be cool if level 5 was added to the game.... so then we can get Yoki Watch and Professor Layton joining the fight. But I don't see that happening.
I'm always 50% right all the time