I'm about a 85% feeling Sonic will be in, with the only reason why he wouldn't be in would be if Sakurai himself felt there's too many third party characters.
Snake I'm a little more concerned about. When Snake was announced waayyy back when he just had an outing on a nintendo system (twin snakes on GC.) He hasn't really shown up on Nintendo hardware except for the Snake Eater 3DS port. Plus there isn't that "omg" factor this time with Snake. Still like I said for continuity, awesomness and the fact that he was my main I really hope he's in the new game.
If we DON'T see a Namco character I'd be shocked. The problem is WHICH Namco character. The whole "gotta be on a nintendo system" stipulation makes a lot of modern choices unavailable (sorry Klonoa and Robbit!) Tekken characters would be boring but Soul Calibur characters (limited to Soul Calibur 2) would be interesting. Pac Man is probably the obvious choice but seeing what was done with him in SFxT i'd rather hope they don't use him.
My personal pick would be Dig Dug or Mr. Driller