Going off of what we know and this drawing from Sakurai these are my guesses.

Returning:
Mario
Luigi
Peach
Bowser
DK
Yoshi
Wario
Mr. G&W
Link
Zelda
Ganondorf
Samus
Pit
Kirby
Metaknight
Dedede
Olimar (Or a player character from Pikmin 3)
Fox
C. Falcon
Pikachu
Pokemon Trainer (Maybe slightly altered)
Marth
Ness/Lucas
Sonic
Cut:
Falco (Clone)
Wolf (Clone)
Lucario (Replaced by another pokemon)
Ike (Replaced by another FE character or cut completely)
Ness/Lucas (Clone)
Snake (There hasn't been an MGS on a Nintendo console since The Twin Snakes, so there's no reason to keep him around)
Unsure:
Diddy Kong (Could see this going either way. I feel that it would not be a big loss to the roster, but he is apart of a popular franchise.)
Toon Link (Initially thought he would return for WWHD, however Toon Link was seen one of the 3DS levels in the background. It wouldn't make sense to have be playable but then have him in the background of a level. My other thought is having Toon Link on Wii U for WWHD, and bring back Child Link on 3DS for OoT3D, since they are essentially the same character.)
Ice Climbers (Are not exactly relevant to any modern series, and Mega Man could easily take their place as the NES representative of Smash Bros.)
R.O.B. (Much like Mario Kart DS, R.O.B. could have been a one time appearance.)
Jigglypuff (I honestly didn't expect the balloon pokemon to return in Brawl, but I could be wrong again.)
New:
Mega Man (ABOUT DAMN TIME)
Villager (Eh?)
Wii Fit Trainer (More eh?)
Namco Character (Since they are helping develop the game, they are definitely getting a third party spot. My first guess is Pac-Man, being that he's their mascot and it would help promote his upcoming game. The Tales director also mentioned wanting to get a character from the Tales series placed on the roster.)
Platinum Character (Given Nintendo rather close relationship with Platinum Games recently, I could easily see one of their characters placed on the roster, since it would also help promote their games. My guess is Bayonetta, although I could see someone from The Wonderful 101 instead.)
Shulk - Xenoblade Chronicles (Given the popularity of Xenoblade, and the fact that X appears to be from the same universe. His chances are boosted due to the fact he's a first party character.)
Castlevania Character (Given that Snake is most likely being dropped, I could see Konami receiving a replacement character from the latest 3DS Castlevania game, Lords of Shadows - Mirror of Fate.)
New Pokemon Characters (Probably something from Black and White or X and Y. Perhaps even a return of Mewtwo with the ability to change into his new form?)
New Fire Emblem Character (I don't know anything about FE, but everyone seems to be guessing Chrom from the new FE game. I could easily see Ike being replaced for someone new.)
Even though I know this probably won't happen, I would love if they added Geno from SMRPG. Square would probably have a more popular character instead, but I will still hope that Geno may end up being a surprise addition.