Sonic will be back. I can't even think of a logical reason he wouldn't. He was pretty much the most requested character across the board. SEGA's not gonna say no and there's no real reason to believe he'd be cut.
Snake I'm not as sure about. Him being in always struck me as weird. Nevertheless, he's not in PSASBR, and Kojima will almost certainly want him back, so unless Sakurai doesn't, he'll probably return.
As for newcomers, I think Mega Man can basically be considered a shoe-in, assuming nothing goes horribly wrong, considering how highly-requested he is. Hell, I don't play Mega Man much and I want him in.
Beyond that, I'm not sure. I seem to be the only person who doesn't really care for the idea of Pac-Man, but if a Namco character makes it in, it will almost certainly be him.
I'd love to see Sora (I'd actually prefer Roxas >_<), but that will basically never happen (despite how well he'd fit in). I also think Ryu from Street Fighter would be a wonderful fit, if unlikely, and it's about time Simon Belmont was in a Smash game, but if Snake stays, I'm not sure how likely that is. I also wouldn't say no to another Sonic character, considering how many there are to draw from. Tails is the obvious choice for a secondary, but I'd prefer Knuckles and I think he'd fit better in a fighting game. Shadow could also be cool if done right. I think it'd be hilarious to see Layton, but I'd imagine they could do a good job. I also think Rayman would fit wonderfully into SSB4, and he does have moveset potential from Origins and Legends...plus Ubisoft has been very supportive of Nintendo with the Wii U. Nevertheless, I don't know if any of their characters, even Rayman, are truly Smash-worthy.
No matter what they do, I don't think it'll go over four third-parties. Maaaaybe five, but that's a stretch.