Geno from Super Mario RPG
Geno is not the most deserving, but he'd be an interesting character. His moves in his game are very unique and fun.
Steve from Minecraft
Crash would be fun to see in the game, as an icon of early Playstation gaming. Imagining him VS Banjo is really cool. I have a good feeling he'll happen!
Infested Kerrigan from Starcraft
This would be an extreme WTF character. I love playing as the bad guys... Ridley, Bowser, K Rool... Kerrigan is the icon of Starcraft, which was a game that appeared on a Nintendo system!
Dragonborn from Skyrim
Ganon (pig form)