I just want uptilt to not suck.
Up tilt doesn't suck; the only real problem with it is that it lacks a back hit-box.
Otherwise, it's good as a combo tool if you hit with two of the three parts of it. One of them has a kill confirm into up smash and confirms into other moves. Up tilt is far from bad.
What if sakurai decides one more patch, and that happens to include another and final Mewtwo buff.. But he can only pick one of these categories. Which one would you pick?
Power - any two moves of your choice now KO 10% sooner, or one move kills 20% sooner
Defense - Mewtwo gains roughly 15% in weight, which is back to 85 weight from melee. In addition, intangibility increase from 9-17 to 8-18 for teleport, and ending lag for teleport is decreased by half a second.
Speed - Mewtwo gains a 10% speed increase in walking and dash. Dash speed goes from 2.05 to 2.25, and walk speed is 1.2 to 1.32, meaning he is now slightly faster than little mac.
Air - All aerials have -2 frames landing lag, neutral air now has a hitbox that goes to the ground(like melee). Air acceleration also increases from .03 to .07.
Frames - Forward tilt starts 4 frames sooner, up tilt starts 2 frames sooner , and down tilt starts 1 frame sooner , Overall FaF frames reduced for tilts between 3-6 as well(6 for forward tilt, 4 for up tilt, and 3 for down tilt) . Down throw throw ends 4 frames sooner, now being able to combo again like it did for Mewtwo in Melee.
Hitbox - Mewtwo's grab hitbox now extends one unit down + unit forward. Basically given the wft and zelda treatment. Up tilt now reliably hits enemies from behind, and back air now hits short characters while short hopping. Cherry on top, shadowball gets its hitbox while charging from melee.
For Power, it seems reasonable. I don't have much to say, but I'd like forward smash and back throw to kill earlier.
Defense
Is it not clear that this character is supposed to be glass cannon? If it doesn't feel that way to you then you might not be punishing right.
Speed
I don't know what to say, honestly.
Air
Making him have better FAF is
1: Unnecessary, his FAF is actually pretty good (better than Cloud's actually)
2: Could make his air game a little overwhelming
I would really like air acceleration though, good idea.
Frames
Making forward tilt faster would be great and all but it's the move itself is probably the biggest reason it's not very good.
Down tilt and up tilt are fine. (The buffs are good.)
For down throw, I dunno, maybe worsen the angle so it can't combo like down tilt can.
If you make it as good as down tilt that would be stupid; Mewtwo should never have a dashing down tilt.
Hit box
I like the grab idea and up tilt too.
Back air isn't meant to be used near the ground.
For Shadow Ball's hit box, there is no reason to add it back.
1. That would make it so that Aura Sphere is completely inferior to Shadow Ball. It's also now just for Lucario only.
2. Shadow Ball is already really good.
I like some of your ideas, but please know that some of this is somewhat overwhelming.