I would like to see more third-party characters, along with the return of Snake and (definitely) Sonic. They really should be iconic within Nintendo's history though. Big games that appears on the NES, for example. It'd be nice to see Simon Belmont, one of the Battletoads (with custom/color choices for Zit, Rash, and Pimple), or Pac-Man... for example. I'd think that Megaman would be a
great choice as well, but Capcom won't even put him in their own crossover fighting games.
More than anything though, I want
real alternate costumes... like Wario had. Sonic should also be Shadow, Silver, and Metal Sonic. Snake should be Solid, Liquid, and Naked. Mario should have Dr. Mario, Link should have Toon Link, etc. If nothing else, it'd be nice to see 8 or 16 bit costumes for a lot of the roster.