Speaking of leaked lists IGN has paid attention to them and made a list of
some of the more prominent characters.They list Pokey, Black Shadow, Alice the balloon kid, Hector from Fire Emblem and R.O.B.
If Pokey's in the game as a playable character Sakurai will have gained a dangerous enemy in the form of a big, hairy Italian.
Black Shadow from F-Zero would make sense as he's the closest thing to a villain the F-Zero has ever had. Since Goroh is out of the question and Blood Falcon is an alternate color for Falcon Black Shadow is the next best thing.
In all honesty while I would welcome the Balloon Fighter (as he would add another franchise to the series as well as an unique fighter) I wouldn't want the fighter to be Alice. She just looks so weird...Plus, the original BF design is more classic and iconic. On the plus side, though, it would add a female character to the roster.
I never played any of the FE games (I only saw SB play Path of Radiance), but Hector would make for a nice addition. We got sword users and hammer users so having an axe user would be a nice change of pace.
Finally, even though it might be out of the question R.O.B could indeed be in the game as both an enemy and a playable character. First, Brawl doesn't have that "WTF" character like Mr. Game and Watch was in Melee so R.O.B could fill that position easily. Second, just like with Tingle and Petey Piranha Nintendo seems to have a fascination with this character. He may have nostalgia appeal but if I'm not mistaken only a couple of games used him and he was a slow and clunky peripheral. But despite that fans and Nintendo love the little guy so don't be all that surprised if he's in the final game.