Character: Mewtwo
Current Tier: F
Improvement: Working special moves.
Results: His Forward B (Confusion) will actually reflect projectiles, serve as a great anti-air approach from SHFFL, Float, Tilt, and SHL abuse, and lead into free grabs or forward airs on floaties, or free down smashes and techchases on fast fallers from the extra stun.
Down B (Disable) can have two possibilities... keep the same criteria of working against grounded opponents that are facing you, but having more sleep time to land a DJC down air or smash at lower percentages and hitting through shields, sidesteps, and rolls.
Or... Make it come out a bit faster, remove the grounded and facing requirement, and stun opponents facing backwards or in the air to be a good edgeguarding/anti-air move against certain characters. I wouldn't even mind if the stun scaled the same as it did now with those changes.
Conclusion: Mewtwo has unique special moves that actually do something. He might even gain respect as a character! Mewtwo's biggest problem is that he doesn't have ANYTHING threatening in front of him so you have to play an absurdly air tight WD/DD spacing game with a character that jumps like Ganon/Yoshi/Falco just to scratch someone until they're high enough to be KOed with up throw.
Even with those changes, he would still be squishy, but have an option to lead to his other finishing moves and combo starters, which would put him somewhere in D tier, probably under Pikachu.
Proposed New Tier: D-Tier