I agree on Kid Icarus and Olimar, I think both could play much differently from any existing characters. I also want to see Wario...he probably wouldn't turn out as unique as Kid Icarus or Olimar, but they could make him different enough to justify his existance, and he really "deserves" to be in the game more than anyone else.
Beyond that, there are tons of characters Nintendo could work with and I'm excited to see who they add, but those are the ones I really want to see in the next game.
Oh, and I totally agree about creating drastically different skins or costumes or whatever you want to call them. I think it might be hard to revert back to some older designs without changing the way the character plays, but it's a great idea. I've been thinking the same thing ever since I found out Dr. Mario was in SSB:Melee.