I will say, given the new info we got today, absent any new revelations, I expect he'd get the most change in a re-rate.
Anywho... Mewtwo.
Popularity: He's very popular with the Smash crowd in both the West and Japan. He's a very popular Pokemon and starred in a movie. He seems a bit more prominent than many Pokemon that are kinda the flavor of the week (or generation as it may be).
Relevancy: Well, it seems he'll be showing up again in the next Pokemon game (and likely again in the anime) with a new look. Recency seems to be a factor with Pokemon, Sakurai seems to value the input of the Pokemon developers a bit when it comes to deciding which Pokemon to put in the game (I assume they have even more influence on the Pokeball item), and their interest is promoting the latest version.
Design: He's already been in the game. He needed a bit of tweaking tho, cuz he was really bad compared to most characters. He will likely have some new moves in the new game (maybe be a Fairy type?), and I assume Sakurai will have access to that if he decides Mewtwo's moves need an update.
Roster considerations: We're already approaching Pokemon saturation, so I can't think that we'd see more than one new Pokemon without seeing cuts. But who knows. I'd really prefer not to have effectively 8+ Pokemon in the game though.
Technical/legal issues: The only issue here is that GameFreak is 2nd party, and they get more of a say in who gets into Smash.
Sakurai statements/rumors/etc.: Sakurai has stated they're considering him. He didn't really want to cut him the first time. He doesn't like cuts, and he knows that Mewtwo still has a fan base in Smash.
However, the new form seems to be an evolution... Or it's an "awakened forme"? I don't follow the current Pokemans, so... Does that mean it will have a new name? If that form were to get in, would it count as Mewtwo? I could see Sakurai going either way in that case. It's possible that Sakurai could pull a Toon Link on us and Mewtwo's evolution gets in under the new name, so it's technically not Mewtwo, but it basically has Mewtwo's move set.
On the other hand, it could just be used as his Final Smash.
Mewtwo chances: 92%
Edit: I am being informed that Mewtwo's new "forme" is not an evolution, and so it would be still considered Mewtwo. I'll trust that. Otherwise I would've given like 73%, since that wouldn't count, just as Toon Link doesn't count as being the same as Young Link.
Anyway, Mewtwo's very popular, was in Smash before, and was only cut due to time constraints. He's on Sakurai's radar and relevant to the upcoming Pokemon game, so it seems quite likely he'll make his return.
Mewtwo want: 53%
I'm pretty neutral on him. I didn't really play him much in Melee, and I haven't been into Pokemon since the 2nd generation, so I'm not very invested in which ones get in. The only ones that I care that much about are Pikachu and Jigglypuff. Pokemon Trainer (1st gen) is kinda cool, I guess. I don't really care who we get out of Lucario, Mewtwo and other possible Pokemon.
Krystal prediction: 40%
This is a hard one. There seems to be a fair amount of support and also a lot of hate here. Combine that with Star Fox already having a fair number of reps, and I think her numbers will be middling. GameFAQs will push her down. But the self-selection bias in voting will come into play and boost her numbers as well.
Nominations:
Simon Belmont x5