I don't know what they'll do with the roster and find it too difficult to predict because based on the three games we've had there hasn't been much consistency or clear direction in what Sakurai wants to do, beyond the obvious of exploring new additions to every roster. Melee took Smash 64's puny roster and more than doubled it with new additions, if a bunch of clones. Brawl culled a few from Melee's roster, worked harder at giving clones more unique fighting systems, and added a bunch of new characters as well.
And so, it's very difficult to predict who will return, who will be cut, if anyone will be cut, or how movesets will change.
Additions to each roster have ranged from the logical like Zelda, Peach, Bowser, and so on. To seemingly forgotten low key Nintendo characters like Ice-Climber, and oddballs like ROB, Mr Game & Watch, Dr. Mario, and so on. Now we have Wii Fit Trainer, a character Sakurai deliberately put in to fuck with fan expectations. And lets not forget how insane it was learning that Solid Fucking Snake was in Super Smash Bros.
So based on that, how do you predict a roster? It's easy to say shit like Rosalina is kinda believable because she's become a bit of a staple in recent Nintendo games with multiplayer focus, and hasn't been discarded since Galaxy ended. Megaman seems obvious now, but people expected him in Brawl first, a game that had only two third party characters while also using the semi-clone trio Fox/Falco/Wolf.
Ultimately I think Sakurai and co's intentions with the roster could literally be anything and thus all predictions and expectations are really just fan lists of what people would like to see, with way too much conjecture when arguing probability, especially for franchises with a tremendously diverse cast like Fire Emblem and Pokemon.
Smash 4 is one of those games where I literally do not know what they're going to do with the roster. I don't know which franchises will make a surprising appearance as part of the playable roster. I don't know who, if anyone, will be cut. I don't know what changes will be made, or why. I don't know how they're going to do Pokemon, or Fire Emblem. I don't know how they're going to treat clones of old. I just don't know, and nobody here does either.
And honestly, for me that's part of the fun. I didn't expect Dr. Mario and Mr Game & Watch. I didn't expect Wolf and ROB. And I didn't expect Wii Fit Trainer. Surprise and unpredictability are nice.