Chef Kawasaki
0.1% Chance - A cool Kirby character, but he's very obscure and irrelevant, without any popularity to speak of. Saying, that, he has some potential for an alright moveset, and stands above other generic bosses or mooks in his series, so there is a minute chance.
1% Want - It'd be a random character. I imagine it'd be fun to play, but Kirby hardly needs more reps, especially random ones.
Mario 3D World Stage
25% Chance - New Mario game that has a bunch of creative levels to adapt, one will surely be in there just to show off the game. It's close enough to release of SSB4 I can see Sakurai playing it or at least knowing about it, enough to put it in. However, there are other Mario stages that could supersede it. If there's a 3D Mario game in the works to be revealed at E3, or if he wants to include a new Mario Kart stage, I can see this game being ignored, because there's no big gimmick to represent in the game (like Galaxy's physics), just the game itself.
5% Want - I'm not a fan of the game and I don't care if it gets a stage or not. There are probably plenty of levels that can be adapted into something fun, but that goes for a lot of things that aren't another Mario game.
Predictions:
1.5% Master Hand
0.5% Playable Bosses
An original character from Smash being playable, or any boss being playable as a focus when there is limited time to make a roster is extremely unlikely. That said if any boss from this series were to get a moveset, it'd be Master Hand.
Goku x5