User blog comment:Wachowman/What are your New Character Thoughts/@comment-24843283-20140901003617/@comment-4115159-20140901190329

What's your logic for thinking that? That's not a rule Sakurai has constrained himself to. that's just something fans have made up. Just because Mewtwo wasn't in Brawl doesn't mean he can't ever return to Smash. That is an imaginary Smash law.

Besides, Mewtwo was indeed planned for Brawl and only never made the final roster due to time limits for the production of the game. So they were thinking of putting him in even Brawl, and now that he has become kinda relevant again he is a fairly likely candidate.