I've actually made very few concrete suggestions regarding Mewtwo. I consider him a rather thorny design challenge because of his reliance on certain sensitive options, but also because of his sky-high skill ceiling and the way different adjustments could force him into undesirable playstyles as his meta develops. This is further complicated by my desire to retain his current skill ceiling through any balance changes, and ideally he'd retain as much of his current playstyle as possible too. While I'm not entirely opposed to certain alternative options, I'd rather not alienate those who genuinely enjoy his current playstyle.
This is my most recent post on the topic iirc:
DrinkingFood had a pretty good idea a few posts later, which was making it so that teleport only refreshes when you touch the ground or grab a ledge. That'd put a stronger element of risk into his gameplay, but it sidesteps a lot of the potential issues that messing with teleport frame data invokes. He also correctly identified a number of places where Mewtwo arguably needs buffs/fixes, as he's got a few dumb bugs and several attacks with hitboxes that straight up whiff in situations where they arguably shouldn't due to his height.
For what it's worth I actually think his range is very appropriate to his current frame data on almost every attack; utilt and uair offend more because of their coverage and chainability than their range at any specific point in the attack. If you were to make an argument about range on any of his options then those would probably be the ones which most warrant a nerf, but I'd rather see utilt simply cover less horizontal space (kinda like a Marth utilt) and do... something about uair, haven't decided what yet. I'm against nerfing teleport's frame data in pretty much any way unless he gets tools to compensate for it; it's too critical to his gameplay right now. I think uthrow should be nerfed, wouldn't immediately object to a weight nerf without considering other factors (I like that the higher weight better reflects his actual properties but it's an option), the real problem with his sweetspots is that they're literally on his ***, and I'd probably leave the tail disjoint as is given his current idle and running animations.