I know 4Chan is a pretty unreliable source for leaks, or anything, for that matter, but let's not forget that Smash 4's final roster was leaked there. I'd give this a little credibility.
Bowser Wii U sounds awesome! I think it's nice to see Bowser finally getting his time in the spotlight. However, if you can't play as Iggy or any of the other Koopalings, I'm not going to be a happy user. Also, it's nice to finally get some info on Mario Maker. I've been wanting some more news on that since the day it was released.
As for the Star Fox Wii U game, I honestly don't know. As long as they can make this whole ordeal interesting and not completely redo Star Fox 64 again, I'll be fine. I think that I'll withhold my verdict on this game until we get more information.
Don't play Fire Emblem.
As for Mewtwo's supposed reveal trailer, I'm not sure what to make of the title. I mean, it's cool and all, and the Brains part is obviously Mewtwo, but who's the Brawn? I would guess that it's someone whose already on the roster, but if it is an extra newcomer, I'd expect a clone of a currently existing character. The problem is, no Melee veteran really fits that description. Dry Bowser, anyone?
Or Ridley, because Ridley is so big.
In any case, this is assuming this is even true, which it very well may not be. As for my certainty on its validity, I want to say 50%, but I think my drive for Mewtwo and a playable Iggy Koopa pushes it up to ~55%.