Here are some guesses at who would be in the next Smash Bros. game based on what I know and who I want to be in Shin Megami Tensei X Fire Emblem.
Super Smash Bros. 4:
Mario: We know he's in it
Samus/Zero Suit Samus: Zero Suit hasn't been confirmed yet, but I doubt they'll make the change.
Donkey Kong: same
Wii Fit Trainer: same
Peach: I really doubt they'd take a "Smash sister" out, people have been asking for a greater feminine presence, I've found.
Zelda/Sheik: Same as Peach, but she'll be Skyward Sword inspired.
Ganondorf: Really doubt they'd cut someone this iconic.
Lucario: He fits so well in my eyes, with his aura increasing his power as he gets hit. Plus, I'm sick of rotating Pokemon in and out.
Snake: Kind of wishful thinking, but Snake was a lot of fun in Brawl, especially the CODECs.
Sonic: Also a bit of a long shot, but also really fun to play as.
Mr. Game & Watch: I just don't see him being gone, somehow.
Falco: I think they'll have 2, not 3 Star Fox representatives.
Captain Falcon: Captain Falcon is like the Chuck Norris of Smash Bros., everyone considers him the epitome of manliness.
Marth: Too iconic to be removed.
Lucina: The 3DS has a stage based off Awakening, plus another "Smash sister" could be fun.
Palutena: I think there will be another Kid Icarus representative, and the audience is already familiar with Palutena.
Meta-Knight: Sakurai made Kirby, if he had total control, Bandana-Dee would be in the game.
King Dedede: See Meta-Knight.
Ghiranim (the bad guy from Skyward Sword, if that's not his name): Another villain would be good, and Toon Link seems to have a new job on the train.
Jill Valentine: Another long shot, but she's appeared a lot on the 3DS, and would fit in well in a Smash Bros. game.
Sorry, I made it only 30, not counting Transformations, so I ran out of space for Earthbound/Mother.
Hopefuls for SMT X FE:
Chie Satonaka, Yukiko Amagi, Rise Kujikawa, Mitsuru Kirijo, Marth, Ike, Lucina, Lyn.
I know, "Persona already shone in P4 Arena," but I want to see those characters appear on a Nintendo system, personally. It would also be cool to see High Schoolers of a present day interact with knights of old.
So that's it for now. Requests, I hope to finish soon.