Keep in mind that the only characters who are in actual danger of getting cut are clones and third-party reps (only reason Ice Climbers and Pokemon Trainer were cut were due to the 3DS' limitations). Pokemon is a tricky question, because they seem to be having a six-party team theme going on since Brawl, which had Pikachu, Squirtle, Ivysaur, Charizard, Lucario, and Jigglypuff. Smash 4 now has Pikachu, Charizard, Lucario, Jigglypuff, Greninja, and Mewtwo, so in order to make room for a newer gen Pokemon, one of them has to go, which is most likely Greninja, because we already have Sheik as our ninja fighter.
But the most likely characters to get the boot in the next game are probably the 3rd party characters (Shulk, Pac-Man, Mega Man), because I don't think that a 3rd party character has ever lasted more than one game.
Shulk is a first-party character; therefore, he is safe. The only third-party rep I see getting the cut is Pac-Man, assuming if the next game isn't developed by Bandai Namco. Sonic has appeared in Smash twice now, so why not have Mega Man stay a little longer as well? As for Snake, with recent news of Kojima leaving Konami after MGSV, getting him back would be more difficult than before, since it was all Kojima's idea to add Snake in Smash to begin with.
And I kinda get the feeling that Robin and Lucina won't make it into the next game. I suppose it's a bit early to tell since Fire Emblem has gained a lot of popularity in the Western world in recent years and a lot of that had to do with Awakening, but I wouldn't be surprised if Fire Emblem is gonna always be represented by Marth+Ike+1, maybe 2 characters from the most recent FE game. I'd really prefer it if Marth was dropped and Robin was made a permanent roster member, but that's probably not the most likely scenario.....
Lucina is most likely the one getting the boot to make way for a newcomer from the most recent title. Robin will be here to stay, because he represents a now staple feature in the series, which is the avatar.
