My favorites and likeliness for another 3rd party character are
Megaman for his popularity and Capcom relationship,
Pacman for Nintendo's relationship with Namco,
Bomberman and Rayman for Ubisoft's support of Wii,
NiGHTS because of the upcoming Wii game and Sonic's entrance
Tails or Knuckles because of Sonic's entrance, etc.
I don't think a FF character would show, but Square did make the DS basketball game with FF characters and Mario characters. Who knows.
I would say that Megaman would be on the caliber of Mario, Sonic, Snake. It's odd but amongst those greats, even Pacman seems like smalltime. Could Nintendo pull a fast one on us and have Banjo? Seeing as MS is letting Rare make games for DS and possibly putting them on the VC. It would also garner more publicity for the upcoming Banjo Kazooie 3 for 360 if MS let that happen.
I would also expect to see Chibi Robo, Olimar, Luigi with a vacuum, Ridley, etc. as first party characters. And much less, possibly offshoot characters like Wind Waker Link, Paper Mario, etc as well with completely different movesets.