oh boy. the "hype the hype" strategy of nintendo is apparently already working. Oh well..
OT: I think they'll keep the roster smaller or the same size as in brawl. This ain't no friggin mugen game. This means we'll see some clones disappear and probably some other characters as well like ROB since he was only sorta relevant for the subspace emissary, then lucas (see ya the next time I replay Mother 3), one of the furry entourage around Fox, Toon Link (despite the Windwaker HD release), Lucario (probably replaced by the poster boy of Gen VI who inevitably is going to star in one of the upcoming pokemon movies) and depending on how cooperative Sega and Konami are Sonic and Snake.
For some dumb reason Marth continues to be the face of Fire Emblem (ugh...), so I guess we'll lose Ike as well.
This means we'll have about 3-6 spots for new characters. Since Namco is on board on this one, there is only one character that has to be in there from their franchises: Klonoa who way cooler than any what's-his-face from tekken anyway.
Other than him, I'm hesitant to do any speculation on characters since I'll probably be wrong anyways but there are some characters I'd like to exclude from speculation from the get go:
Megaman: Seriously, what's the blue bomber going to add to SSB since gameplay-wise he's like the unholy bastard offspring of Samus and Kirby.
Shadow the Hedgehog (or any of the other massive amount of annoying woodland creatures from the sonic universe): No... just no.
Bayonetta: Okay, to be honest, I'd love to see her in there but somehow I can't really picture her in any family-friendly game without toning down her overly-sexualized character and frankly, who'd want that?
Gameplay-wise I hope they'll give the stages more character than in Brawl. I'm one of the few who loved the platforming levels in melee (pokefloats is still my fav stage) and I'd love to see more of them who provide and challenge while being forced to leap through the air like a silly goose. Also, the single player campaign should feel less fanfictiony this time around. :/
Oh yeah, and don't screw up the connectivity between the 3DS and the Wii U.