I'm just going to do all the vets for fun.
-Wolf is easily the most likely. He has everything going for him, and if Sakurai doesn't want to make anymore "clones" he could just make him even more different then he already is. Seriously Wolf's specials are only kinda the same, if you change those, no issues.
-I actually don't see Snake as all that likely, like everyone else does. Konami firing Kojima really eliminates any chances he had left in my opinion, seeing that Sakurai only put him in Brawl for Kojima. Even if he was to return, it probably would've been planned for around the release of Phantom Pain, so I think Snake's ship has officially sailed. He feels less likely than the Ice Climbers to me, honestly.
-Not sure about Squirtle/Ivysaur, though I feel Ivysaur has an actual chance to complete the starter types. Not really all the big on either of them though. I love Ivy's kit, but really hate four-legged characters. I guess I'd put Squirtle around Snake's likeliness, because his type has been replaced with Greninja, so unless he gets a huge push, I just don't see him all that likely.
-I don't think Pichu/Young Link don't have a chance, at least I hope they don't. They bring nothing to the table. I know people hate this phrase, but they'd both be a waste of a slot, or more specifically a waste of development time, effort, and money.
-I was really down on the Ice Climber's chances for a long time, but I feel that there are creative solutions out there to make them work. Seeing that Sakurai obviously has a personal love for them, and that he's refused to take the easy way out on them, I feel like maybe something's in store for us.
So here we go.
1. Wolf
2. Ice Climbers/Ivysaur
4. Snake/Squirtle
6. Pichu/ Young Link