and
are not true villains, but there's more than enough history in their games to pass off any of them as a villain.
is probably the worst of the four. He's teamed up with Fox before, but he has a pretty long history as a criminal who kills and destroys.
would be next. Not necessarily evil, but he's not nice either. Very rarely will any of his heroism not involve his own personal dreams of getting rich. He's essentially a jerk who loves money.
is similar to Wario, minus the ridiculous selfishness and crude humor. Can be a glutton at times, but he's gone out of his way to help save Dream Land on multiple occasions, most of which don't involve a personal incentive aside from protecting his home.
even when you include the anime, Mewtwo is not evil. His goals have never been strictly malevolent, and his awful actions are typically done because he genuinely believes he's doing the right thing. Despite this, Game Freak continues to push him as a "final boss" of sorts. Not a villain, but his role as a super powerful and incredibly dangerous Legendary makes him come across as one, even though he should be classified as neutral at worst.
If Sakurai wanted to expand the list of villains, any of these four would do. Maybe not as good as Bowser or Ridley, but they'd work regardless.
I don't understand it either, but I guess it really doesn't matter how I feel. Clearly a lot of people think he's likely, so I'll try to be reserved with my disapproval and let them support their character.
Even though I don't like Geno, I don't want to see a popular character get ignored either. If he gets in, I'll be disappointed and confused, but at least his inclusion was justified by fan demand.