Yeah, really...
To this day, I still wonder just how Mewtwo would have looked and performed in Brawl...
I mean, what would his Final Smash have been before Mega Evolution and Psystrike??
I usually wonder most about the moveset and model. I kinda wish that Sakurai would inform us, or maybe even make a premium Brawl copy with Mewtwo in it for the fans (since wifi is dead, so personal content or social content doesn't make a difference).
For some reason, I always have thought that he would be good, though similar to Melee, while also having a wonderful model (almost like a vision, or perhaps a collection of desires that have manifested themselves into one)
You also have to remember that Smash 4 Mewtwo isn't a useful reference for how Brawltwo would've been because:
His animations (and possibly hitboxes) have been altered to make him face the screen more
There was a huge focus on buffing veterans, however, Mewtwo wasn't buffed, just changed, possibly because this would be his first instance of being a veteran, therefor they didn't have an already-established Brawl moveset to work off of.
The animation style as a whole (and hitbox data) was different from in Brawl, so Mewtwo could've still had his Melee-animations
Mewtwo would probably have been buffed from Melee, but could've also been nerfed, and because we don't know, we also don't know what happened to Smash 4 Mewtwo in relation to Brawl, because Brawl Mewtwo never existed
Sakurai seems to focus more on characters that are in the game longer, so if Mewtwo were in since 64, 4 example, by the time Smash 4 came along, who knows how different he would've been. (don't believe me, look at Mario, who has gotten many changes throughout the series, then look at Sonic, who hasn't gotten those many, then look at the Smash-gaps between them)
Hitboxes in Brawl are usually bigger and less realistic, which means that Mewtwo's range could've actually been buffed.
His Up-throw and Back-throw probably would've retained their power, Down-throw would've been a better move, and F-throw probably would've still sucked.
And lastly,
Mewtwo's two good for Brawl.
Not as sick as the one he has now, of course.
Honestly, I don't know if they could have done anything more awesome with his Final Smash. They nailed it so perfectly....Mewtwo Y AND Psystrike.
]I don't know Y they had to make it Mewtwo Y. I hate it when they appeal to the majority, because what Nintendo doesn't realize is that there was a time when people hated MMY, yet by advertising him and giving him spotlight, people grew to love him. But if they never do that same thing with MMX, then of course people will dislike him. This was a dumb move on Nintendo's part, as once you're on the throne what else is there to do, MMY? Also, Psystrike should've been more like how it is in the games.