Meh, my shot at making a list:
Little Mac (obvious, popular character, also revived with the Wii version)
King Hippo (controversial, but could fill the obligatory super-heavyweight spot, also revived in Wii version)
Takamaru (classic, Nintendoland brought him back from the dead)
Bowser Jr. (one of the only Mario characters who has merit to getting in IMO, not something stupid like Toad or Waluigi.)
Girahim (major character in SS)
Palutena (Uprising)
One to two of either Viridi, Medusa, or Hades (Uprising, once again)
6th Gen Pokemon (impossible to predict atm)
XXX-Gen Trainer (also impossible to predict, I mean back before Brawl who would've said that Ivysaur would be getting in?)
Mewtwo (since the fans seems to want him back)
Obligatory Ike replacement (since no one can seem to agree who it'll be)
Any range of Namco Bandai characters (Pac-Man or a Tekken character most likely, but they have other properties too.)
Xenoblade-character ( I haven't played it yet!)
A Golden Sun character has a good chance of getting in
Other Stuff to note:
Ganondorf stays (maybe since he lacks an appearance besides OoT 3D he can finally use his sword??)
Toon Link stays (Spirit tracks)
Mario and Luigi gains his Spin-thing from Galaxy as his down-B
I think Star Fox is on it's limit for characters
Roy has a chance to get in using his redesign from Awakening
Donkey Kong gets no Characters (controversial, but none of them appeared in DK Country Returns)
Ridley does not appear as a character (very controversial, the character is gigantic and can fly, so I don't see him coming in, I also feel like that If he was going to be a character he'd be one by now. I could be very wrong in this one however.)
Balloon Fighter has a chance if getting in (he was considered for Melee you know, and was in Nintendoland)
Waddle Dee will never come in (c'mon guys, it's like trying to add a Goomba, Kirby already has enough characters)
Animal Crossing will never have a character
I think a Baby Mario and Luigi character would be cool (two in one, but they hold hands all the time so it's not an IC situation, probably not happening but a man can dream)
Sonic and Snake stay, and may add some more of their own characters
Samurai Goroh doesn't really fit as a character for this game IMO
Some of the eShop's first-party's, such as Pushmo and Crashmo, Dillon's Rolling Western and the Samurai one could possibly get in.
Anyone with a brain would realize how ridiculous it is to think that Waluigi or Toad would even come in, they have absolutely no merit to being characters and both of them have really done nothing worthy of being a character for SSB in any mainstream Mario game (Hell, Waluigi isn't even in one of those, his whole character is competely just a spot-filler for Mario sports games, I'm glad Mario Kart 7 got rid of him.)