Phoenix Wright vs. The Hulk would be...a bloody mess, but it would be an amusing bloody mess.
Dante is a must, probably Nero and maybe Virgil. Soki from Onimusha would be cool. I wouldn't mind seeing Bass, either.
On Marvel's side...having some of the newer kids on the block would be interesting. Having The Sentry or the Young Avengers would be cool. Having Zombie outfits for a couple of characters would be fun; and hey, if they do that, they can include Ash Williams! Tell me that wouldn't be badass!
Apologies Abound