I disagree with the Mewtwo thing.
Mewtwo's arms aren't any shorter than the arms from others, and his size can't be a problem, he's not that big. Bowser is bigger and Ganondorf probably too. Mewtwo can be resized like everyone else, and like it was in Melee. I don't see a problem with this.
He was so hard to use in Melee, because the developers made him like that. It could've been anyone else too. And Mewtwo is also built like a fighter. Not as much as Lucario, but still. And I don't see a problem with a more psychic character. Zelda is somewhat like that too, and Lucas and Ness.
I don't see a problem with Mewtwo's tail, it's not that long and there are attacks that have more reach. And it wasn't the ultimate weapon too, as you see on the tier list.
I really hope Mewtwo makes it back into the next Smash, but I'm not sure if it'll happen. Maybe there'll be a new Pokemon generation before the next Smash is here, and the new Pokemon will be from there. It makes Mewtwo's chances slim, and that's really sad.