For buffs, he should be faster then Pikachu, he is way faster then Pikachu is in the source games, he shouldn't be as slow as he is. Let's forget the anime, Mewtwo's ingame stats are over twice as high as Pikachu's in the games, Mewtwo needs to outclass Pikachu and Charizard in every way possible in order to be respectful to the source games and considering Pokemon is Nintendo's 2nd highest selling series that's kind of important. If Mewtwo is weaker then Pikachu again...
Anyways, have you guys considered that he will have custom moves as well? I'm going to list some based on what he can learn in the games:
Original B: Shadow Ball (Should work like Project M one)
Custom 1: Calm Mind (Works just like Project M Lucas' B Move, in Pokemon it is used in a similar way)
Custom 2: Power Up Punch (Similar to Falcon Punch except Mewtwo's next Smash deals extra damage if it connects)
Original Side B: Confusion (Should work like Project M One)
Custom 1: Psyshock (Should work like Koopa Klaw custom, but breaks shields)
Custom 2: Guard Swap (Must connect, Swaps defensive stats except for Height with opponent, for example gaining Bowser's defense and weight and even armor and Bowser has to use Mewtwo's stats)
Original Down B: Disable (Actually get rid of Disable it's terrible, it may even be worse then Rest)
NEW Original Down B: Power Swap (Must connect, Swaps offensive stats with opponent)
Custom 1: Double Team (Works like Lucario's Double Team except counter attacks have a Shadow effect)
Custom 2: Charge Beam (Chargeable Beam of Electricity YES HE CAN LEARN IT, uncharged has terrible range, normal is normal and after a lengthy charge it deals massive damage in a straight line across the screen)
Original Up A: Teleport (Should work like Project M one)
Custom 1: Super Teleport (Has better recovery but Mewtwo is helpless afterwards)
Custom 2: Multi-Teleport (Teleport doesn't go as far, but can be used 3 times similar to ROB's recovery in Project M