Master of the House said:
Still, Sakurai has to know that Mewtwo was garbage before. he'll probably buff him. He's done it before with Kirby going from Melee to Brawl, and the opposite was done with Meta Knight between Brawl and Smash 4.
But then again, Ness was trash up until Smash 4.
Link, Mario, Samus, and Ganondorf were totally OP in Meele, and they hit the nerf hammer in Barlw. And in Barlw, Samus was obviously too broken after the transition so it didn't get any real nerfs or buffs. And Zelda's terrible unbalance was also a common citation by fans, so in Smush, she got a useless special move and she has the same mobility as a cardboard.
The Smash development team doesn't know appropriate buffs and nerfs. Not saying that Mewtwo will necessarily be nerfed further, but I guess anything can go.