1. Mewtwo
It's rather obvious a choice, as Mewtwo is the most popular of a choice and the fact that Doc Mario is returning, gives us hope.
2. Vaati
I can count Vaati as he does have his own uniqueness in the Zelda series, and is the only secondary main villain to have appeared in more than one game (2 games), and is, as well, one of the main villains in his own game (Four Swords). He's appeared in three total Zelda games as a villain, so he's pretty much more likely, in my opinion, than Impa or the rest.
3. Leon S. Kennedy
Probably the only feasible M-rated 3rd party character that could make it into the game, Leon could sport an arsenal similar to Snake, but with new attributes. Now even though he's a Capcom character, and we have a representative for it, I still think Sakurai looks more into series for third parties rather than a third party company representative overall. Plus...let's be honest, Leon's appeared far more than Snake did, so if anyone can replace Snake, it's him.
4. Impa/Zant/Young Link
Now for Impa to be in, yes, it would add a good new Zelda representative, as she's the most likely Zelda character, as well as another female fighter. Of course, Zant could also be a character in the game, seeing as he has appeared in Twilight Princess, just like how Ganondorf, Link and Zelda's looks are based from, giving him better credibility.
Now, I wouldn't count out Young Link just yet. If they're returning clone characters with different capabilities and aesthetics, I think Young Link should obviously have a returning spot, albeit similar to his Majora's Mask counterpart, in which he has the Gilded Sword, is heavier, and has a new arsenal overall.
5. Sora
While as highly unlikely as he really is, Sora still has a shot for the game, though it's questionable whether or not he could be in the game. I still have hope.
These are my opinions, so take them lightly.