I don't even think movepool/set was ever an issue for Mewtwo. All other Pokemon have several moves from TMs (even outdated ones, like Skull Bash/Submission on Pikachu) and take "creative" interpretation from elsewhere. I think Sakurai just struggled making Mewtwo a fighter since at the time, Mewtwo was primarily seen as a Psychic-and-only-Psychic character (...kinda get that feeling from Lucario, albeit with Aura), hence why so many of his moves seem to rely on his physical attributes (every tilt being tail move, for example) so as to not make him ridiculously broken as in the games, or rather an actual character that can be actually fought. Anything short of S Tier would have felt disappointing, honestly, hence why people are so hard on the Mewtwo that we got.
As I said earlier in this thread, if Sakurai was basing Mewtwo off of something like Origins this time, I don't think he'd have any of the issues/get any of the complaints he did in Melee. Regardless of Psychic hax, Mewtwo is shown running, being agile/sturdy (hand-to-hand combat with a Dragon, despite losing, is impressive) and does so while showing off its signature classic moves, which by today's standards are incredibly sub-par (Swift was even weaker in Gen I, since it was a physical Attack move).
Also Drain Punch is the best move for X, though that's obviously with Pokebank. Maybe there will more move tutors down the line.