My brother is convinced that Deoxys and Lucario will be playable characters.
Deoxys would make a good candidate, if you think about it. He could utilize all four forms while in battle, and it would look ultra slick to make him change forms flawlessly. Or maybe have it like a Sheik transformation, and input commands in battle to swap forms.
Lucario because he was the star of a movie and he can actually fight.
And somewhat linking to the topic of Soul Caliber, I think Ditto would make an excellent addition. Ditto would be Soul Caliber's Charade; every round yields a new character. In Smash terms, Ditto could be someone different every time you respawn. It would require ultimate strategy to use Ditto well since you'd pretty much have to be good with everyone in order to win. I think it would also be interesting to have Ditto randomly transform into random unlocked characters every minute or so (an alternative to every spawn transform?), the exception being if you are recovering (so you don't end up screwing yourself over).
Not to mention that if Ditto was in, I think less people would complain about the fact that Ditto is a Pokémon and concentrate on him being everyone literally at the same time.
And Nintendo actually owns Ditto. So there's another point for him.
Ditto for Brawl