Highly unlikely, and may never happen, but would still pwn
Chuck Norris
Mr. T
Captain Lou Albano (He'd make an interesting addition, seeing as he played Mario for the SMBSS)
Missingno. (I know Nintendo frowns upon glitches, but come on! Missingno. has made a name for itself just by giving trainers nigh infinite stock of 6th slot items)
Darth Vader (Quit denying it. Vader's presence would pwn)
Captain N
Luke Skywalker ("Luke, I am your father.")
Joanna Dark (Sadly, it'll never happen since Rare's with Microsoft now)
Conker
Has better chance than above list; very popular, but chances range from unlikely to almost likely
Fawful (He'd be the mustard to the sandwhich that is SSBB! If he does not appear in SSBB, I'll have disappointment)
Geno (Another very popular Mario RPG character, but Square will have to give Ninty permission first)
Vaati (His TMC form would make a great addition to the Zelda Team)
Bomberman
Waluigi (As pointed out, if Wario's in it, why not have Waluigi?)
Baby Mario/Baby Luigi (Like the Ice Climbers, they'd be two characters controlled by one player)
Tails (If Sonic makes it in, why not add Tails?
Wolf (Add more Star Fox characters)
Krystal
Pac Man (Wacka Wacka Wacka)
Megaman
Raichu
Pretty likely
Toad ('Nuf said)
Sonic (Who wouldn't want to finally see a true Mario vs. Sonic match? Due to the popular demand, Sonic will be in the bag once Sega gives permission)
Drop out
Pichu (I don't like Pichu as a smasher because his electric attack hurt himself, and Pikachu is light and quick enough. Have Pichu be replaced by Raichu, who'd be a slower, heaver, more powerful Pikachu.)