Well... Not everyone thinks he's returning...
I'm seeing quite a mix of responses from people in the RTC thread. Some think he's still a shoo-in, others just don't think he's going to happen anymore, and a lot are just iffy about the whole deal. Kind of reminds me of Ridley's situation, actually. I guess Mewtwo is more positive, though.
Because you have the people who think "slots" actually mean something and those that don't.
Combine that with the people who think popularity counts for something and those that don't. Then combine that with the possibility of dislike potentially biasing scores, and all of a sudden you have the chance for incredibly low scores.
The way I see it, Sakurai isn't the kind of developer that is going to leave out the most wanted character in the world for the sake of the character select screen looking nice. I say that because he bothered to stop whatever was being done with Brawl at the time to include Sonic. If he's willing to go to such great lengths for not only a character that had to be included with all the things that a new franchise needs (a stage, trophies, stickers, music, what have you.), but a
third party character to boot, I think Mewtwo's in good hands. Even if Mewtwo is possibly not as popular as Sonic was which I can't say for certain either way.
Either way, Mewtwo's situation is
definitely more positive than Ridley's. Ridley's popular, but Mewtwo is...well, Mewtwo.
And also Mewtwo isn't potentially viewed as better for a non-playable role since he's already been playable.
That's another thing entirely right there, actually; he's already been playable. Having even some moveset details already present even just in idea-form is a plus from a development standpoint.
Mewtwo still is one of the most likely characters. Palutena hasn't trumped him, she's only joined his ranks. From my perspective, the only characters that
beat Mewtwo are some of the unconfirmed Brawl veterans.