This will be listed by type of game.
Super Mario Characters
Mario (confirmed)
Luigi: A given to be in Brawl
Peach: Another given
Bowser: Still another given
Bowser Jr.: He has become popular at Nintendo, since they use him in all of the new games like MK:DD and New Super Mario Bros., he will most likely make the cut.
Donkey Kong Characters
DK: Another Given
Diddy: As DK's best buddy and a staple to the DK series as much as DK himself, he's the next DK character, over all others to be added.
K.Rool: Brawl needs more villains, and what better villain than the giant walking croc with the funky eye himself?
Yoshi Characters
Yoshi: Another given
Star Fox Characters
Fox: ANOTHER given
Falco: As the second most popular Star Fox character, he is likely to return.
Krystal: The new blue fox in the series, Nintendo has made her a main character, and she could be a unique one with the staff's powers
Wolf: As Fox's rival (Not Andross, he’s too huge.) and another villain, he is the next Star Fox character to be included after Krystal
Wario Characters
Wario: (Confirmed)
Waluigi: Before anyone starts whining, wait. Waluigi is a popular character (Like it or not) and he could also take moves from the popular Mario Party, Kart, and Sports series.
Metroid Characters
Samus/ZS Samus: (Confirmed)
Dark Samus: More clones you ask? No way Hose, If you think DS would have a cloned moveset, you need to play MP2 some more. Using all of the powers from the many battles you have with her, she could be unique if done right.
Gorea: The answer to the "Which MPH hunter?" problem. Gorea has all of the powers of the other hunters, making it fair to every one who wanted one specific hunter. That, and his speed, (Yes, he is fast, he can outrun Samus.) would be a welcomed addition to the roster.
Ridley: See McFox's explanation in his thread.
EarthBound Characters
Ness: One of the Original 12.
Jeff: Ness's best friend, and the next EB char to get in.
Kirby Characters
Kirby (confirmed)
MetaKnight (confirmed)
King Dedede: As Kirby's main villain, he would be a unique character, using a mallet.
Pokemon
Pikachu (Confirmed)
Jigglypuff: One of the original 12
Deoxys: A third season representative
Mewtwo: A first season legendary representative.
Ho-oh: As a giant bird, he would be extremely unique, plus he's the best 2nd season rep out there. (In the sense that there's only one.) (He's better than Lucario, stupid blue dog.)
F-Zero Characters
Captain Falcon: one of the Original 12
Samurai Goroh: Capt Falcon's nemesis, and a samurai at that.
Legend of Zelda Characters
Link (confirmed)
Zelda/Sheik: A given
Ganondorf / Ganon: Link fought these guys for a long time, as his main villain, Ganondorf should stay, and get his pig form as a transformation.
Skull Kid: A character many have requested, and which I think is a good idea.
Fire Emblem Characters
Marth: As the first FE character, he should return.
Roy: After his appearance in Melee, many people came to enjoy him as a playable character.
Hector: A unique FE character, since there are not axe fighters in Melee.
Custom Robo Characters
Ray 01: A used to be Japan-only character, after Custom Robo was released in America, it was a hit, and the could also use him to advertise for Custom Robo DS
Rahu: As Ray 01's nemesis, I don't see why not to include him.
Random Other Characters
Ice Climbers: Another fun character, and the only dual one.
Stafy: A Japan-only character, but as we saw after Melee came out with Marth and Roy as playables, the fire emblem series was released in America afterwards, so the same could be done for Stafy.
Mr. G&W: As the first real Nintendo character, he should keep his place on the roster.
R.O.B. (NES Robot): A historical character that Nintendo hasn't forgotten about (Mario Kart DS) and being a robot, a moveset could easily be made.
Pit (Confirmed)
Mach Rider: Another historical character, and from a hard game, he should be included.
3rd Party Characters
Snake (Confirmed)
Sonic: The most requested character for SSBB.
Geno: See what McFox has in his thread.
Godzilla: His games have almost all been on Nintendo consoles, and he has an exclusive game coming out for the Wii.
(Yes, I know there are 50 characters, so what.) (And I hate Lucario, that’s why he’s not included.)