Upon reading some of the other posts, I wouldn't mind having more Metroid, Kirby, DK, or Starfox characters. the issue though would be that with Metroid, Samus was the primary character, so other then the huge bosses or the Space Pirates, there arn't too many characters to draw from that. With Kirby... Kirby ate everything, so drawing too much from that game is sort of an odd choice. DK would mainly produce copy characters, as would Starfox. That goes for adding Wario and Waluigi, unless they found a good way to make them more unique then Mario, Luigi, and Dr Mario.
Although along the lines of Mario game characters, Mallow and Geno from SM:RPG would be nice. A Battletoad would be cool because, like was said before, they would stomp a good bit.
And after reading someone elses post... I would KILL for Black Mage to be in it... ala 8-Bit Theatre.