honestly, i'd be surprised if that were it. I 100% expect new characters in the game (aside from those two), and possibly a new villain, which could mean a different story mode. I only think that because porting a game from Wii U to 3DS seems dumb, considering the 3DS is inferior to the Wii U (see Smash for example), so if it's just a porn with new characters, and a handful of new features, that seems pretty stupid, so I'm expecting a new story mode. and of course, any new 3DS characters would obviously be ported to the Wii U, too
also, even if your favorite character isn't in the screenshots, they said at E3 all characters, including DLC characters will be in the 3DS version.