I find it interesting that Greninja has two moves that allow him to vanish into thin air. That was definitely one of Mewtwo's exclusive characteristics in Melee. With the Brawl-style air dodge coming back, it makes me wonder if Mewtwo will still vanish for his air dodge. I still expect him to vanish in every other instance, though.
It's been ages since I last played Melee... I know most people were underwhelmed by his moveset in that game, but I feel like, with all the great changes we're seeing to other long-time characters (Bowser, Samus, Pit, Sheik, and others), a few power/weight tweaks and a couple new moves would be great for his return.
Mewtwo's run, aerials, throws, item use animation, Confusion, Disable, Shadow Ball, Teleport, dodge rolls, and double jump are all very fitting. I want all those to return. Confusion definitely needs a buff, though, and I wish Disable was faster. I think a faster jab, a new dash attack, and maybe a new up smash would do him well. I can't even remember what his forward tilt is... Wow. It's been too long. I know I like his down tilt because it's good, but I think they could make a cooler one.
And while I know Mewtwo doesn't actually learn Shadow Claw in Pokemon, it was one of his few powerful moves. For that reason I'd hate to see it go, but if they replaced it with a more fitting attack that still had KO potential, that would be fine.
Oh man, and his "balancing on the edge" animation - that has to come back! Mewtwo just does not give a blip, and I love it.
Maybe if they make Confusion do some damage, Disable could be replaced by Barrier! Man, that'd be great! Eveyone knows about Mewtwo's love of psychic Barrier attacks. Strangely absent in Melee. If not as a B move, for his Final Smash he ought to fly around with a Barrier up (Mega Evolved), blasting people with Psystrike. No more holding back!