Well, I have to say that Sonic chances are looking really good since he has been in alot of Nintendo exclusive material as of late, as for Snake; I'd say that if Kojima wanted him back in then Sakurai would let it happen since the negociations between Sakarai and both Sega and Kojima are already established, it really wouldn't be so difficult to include them if a current contract or agreement of sorts exists between both parties already, of course it's not guaranteed to happen but I see it as a high probability.
Maybe even acquire a new 3rd party like Megaman or Slime since tbey both seem likely as well, Capcom already and probably has relations with Sakurai according to a recent article where it stated Sakurai was talking with Capcom, the only reason I would consider Slime as well is seeing that Sakurai seems to favor the Dragon Quest series, more in particular DQX which he recently aplauded, not to mention Slime as of late has been in many Nintendo exclusive games even with an upcoming Fortune Street that will feature both Slime & Mario in the same game, I find it as a high possibility.
There's always Professor Layton as well since Level-5 does show interest in adding him to Smash Bros according to a recent article as well, as for who is more likely to appear? I have no idea.