Rosalina and Megaman both got CGI trailers in the same style as Greninja/Charizard. Wii Fit Trainer, too, even though her "trailer" was only about a minute or two long. Villager also got about... 30 seconds or so. They were all done in the same CGI style.
Since there isn't likely to be another Pokemon newcomer unless they boot out Jigglypuff (and I'm not convinced Sakurai wants that), If Mewtwo was going to get his little hurrah in high-def glory, it'd have to be for just him (as much as I'd like it, that's not likely), or as a group reveal with at least one other character, like Charizard.
Who would be paired up with Mewtwo? Well, I think another Pokemon isn't likely. So I see two plausible scenarios: either in a "veteran" trailer showcasing several returning fighters (where Mewtwo would undoubtedly recieve top billing, if there was no newcomer afterwards); or, as the opening act for a newcomer of some sort. But mixing and matching franchises being revealed? Seems odd, huh? I always did like the "villain" angle, even though Mewtwo isn't a proper villain. But he does look the part, and there are many highly desired villains for this Smash. So, I could see something like that, too.