I have pretty high doubts Mewtwo will ever overpower the likes of Diddy Kong, being OP enough for everyone to stick to for a long time.
If I could assume where it'd be on Eventhubs' list, I'd probably say mid-high. If higher than that, then I'd be fine with any rank up to #7. If lower than mid-high, then I'd prefer anywhere down to #35. Not saying that I wouldn't play Mewtwo if it got outside of these boundaries, but I'm saying that I feel like it'd be in an overall "safe" spot between 7-35. Possibly would be even safer if between 10-35.
It'd mean that it's likely good, but not too over nor underpowered to keep tryhards people playing to win from sticking to it, or it from requiring too much attention balancing-wise in the future.
Then again, regardless, people will complain in either direction about the guy, and they likely won't even be the people that supported Mewtwo's release originally. There's gonna be people complaining that the guy should be nerf-hammered like they currently do about Sonic, or there's gonna be people going like "wow it hasn't even changed from Melee, needs more buffs" (even though, you know, it's obviously buffed). That's more my problem with the inevitable flood of Mewtwo. (It does have its positives, though; don't get me wrong) It won't necessarily be the decrease in character playing that becomes a problem, it'll be the possible chaos within a minority of the community, outwardly judging Mewtwo within the first week as if they've figured out everything there is to the Pokemon.