Now for this, i'm actually thinking about this logically, i'm not just going to pull out characters left and right from any popular franchise.
CHARACTERS
NEWCOMERS
Balloon Fighter
Ah, good old Balloon Fight, I remember the good old days when I used to play it for hours and hours. Nintendo seems to have not forgotten about their old franchises adding in such characters as the Ice Climbers and Pit. I couldn't think of any other worthy 8-bit character.
Mega Man
Ah, I know you're going to be like "NO WAI MIGA MNA KAN B IN THIS". But i'd like to say this is a MAYBE. Two third party characters are already in and IF another third party character would be in it would be him.
And i'm talking about the original Mega Man by the way, not any of that X, Legends, Battle Network and Star Force crap.
King K. Rool
The big bad guy of the land of Donkey Kong, not always under his casual name but he's still the guy that you fight at the end.
Black Shadow
Now I would of picked Samurai Goroh as the next playable character from F-Zero. But now that he an assist trophy i'm going to have to go with the next logical choice.
Cel-shaded Link
You know the version of Link from Windwaker and Phantom Hourglass? Yeah, him.
Basically the way I see it they'll throw him in instead of Young Link, with a brand new moveset.
VETERANS
I'm just gonna list them, with new movesets or not. Why they're there is pretty obvious.
Luigi.
Captain Falcon.
Ness.
Jigglypuff (I hate Jigglypuff as a character, but never the less i've got a feeling it will return).
Ganondorf.
Mr. Game & Watch.
Sheik (as a separate character or Zelda's B+down).
BOSSES
Non-playable bosses that i'll think you'll encounter during The Subspace Emissary.
Ridley
Now a lot of people think that Ridley would make an awesome playable character, and i'd have to agree with them. But as I think logically about this i'd say he'd serve better as a boss then a character.
Liquid Snake
Whether you fight him as he is, or fight him in Metal Gear REX, I know he's gonna be in there somewhere.
Dr. Ivo "Eggman" Robotnik
Think about it, who's that guy that Sonic's always trying to stop in almost every single adventure? That's right.
So those are my predictions.
If i'm right, that's cool. If i'm wrong, that's cool too. Just putting my thoughts out there.
Edit:
I was right about Sheik, hahaha suck my dick.
Mad-Mardigan
Skate from Streets of Rage
Trust me
Carbonwater
Considering it's a Sega game.
They've already got their guy in there.