There are so many 'Maybe' characters, I'll make a new list for them.
MAYBES
Geno Seems pretty likely.
Daisy Peach costume or AT at best.
Doc Mario I seriously doubt the cloniest of the clones is returning.
Falco(plz) Seems pretty likely, IMO.
Dark Samus If Metroid gets more than 2 reps, then count on it.
All MPH Hunters ATs at best. There's no way to distinguish who is more important, and there's no way they'd include all six so...not happening. Ridley's first, then DS, but Metroid ain't gettin' more than 3 reps (sorry to say).
Mallow AT at best.
Baby Bowser Maybe Bowser Jr., but Baby Bowser won't appear at all. I'm quite confident in that.
Balloon Fighter Very likely considering his appearance in the Melee beta and the fact that Sakurai likes retros.
Olimar You'll see him. Count on it.
Zant Nope. Not important to the series, appeared in one game, and there are at least a couple better choices for reps of Zelda (especially villain reps).
Midna ehh...people say it's possible, but I say AT at best. Same story as Zant. (not important/one game/better reps)
Vaati Seems possible. I'm split though. This character has made 3 appearances, though; so that shows some importance to the series. It's possible.
Black Knight I'm not sure about this one. Personally, I'd say no because I think Marth will return and they need a different type of rep for the FE series. I don't see three sword-wielders from FE in the game, but that's just me.
Kind K Rool Same story as DS to the Metroid series. If DK gets more than 2 reps, count on K. Rool, otherwise, no. I personally don't think he will just because there are better choices of characters, even though I'd like DK to get more rep.
Megaman Seems pretty likely as the mascot of Capcom. If Capcom is one of the 3rd parties repped, MM will be in.
That's all for now, feel free to add more