Here's a list of characters I'd like to see in the next SSB.
High Priority
Megaman
Bomberman
Knuckles
Medium Priority
Bowser Jr.
Protoman
A couple Chrono Trigger characters would be nice (Crono, Frog, and Robo would all be awesome)
Baby Mario & Baby Luigi (whether separate or as an IC-like team)
Tails
Paper Mario (reeks of awesomeness, another 2-D character like G&W)
Female Pokemon Trainer (would have different, less orthodox Pokemon choices, but still from the first installment, maybe like Sandshrew, Nidorina, and Butterfree to correspond roughly with the other Trainer)
Low Priority
Shy Guy (I know it's sort of a stretch, but c'mon, honestly, he could have an awesome moveset, using different guy-types like Pyro Guy for his Down-B and Fly Guy for his Up-B, and do I hear General Guy as a Final Smash?)
Banjo & Kazooie
Sword Knight & Blade Knight (separate or as an IC-like team)
Birdo (I have nothing particularly for or against Birdo being included, she just seems to fit by now)
Biospark (Ninja enemy from Kirby, where HAL took the wall-cling from in the first place...)
Assist Trophies That Should've Been Playable
Shadow (duh...)
Waluigi (they could think up something for him)
Knuckle Joe (seriously, this guy rocks, for a moveset, check out my post in the "Make Your Move 2.0" thread)
Yes, I'm a Kirby fanatic, so I'm probably including too many Kirby characters for my own good. Before you hassle me, I am aware of this.
And for the record, I'm actually against putting Ridley in. I feel he'd have to be watered down a lot, and that he doesn't fit the roster very well anyway. Just my two cents...