Yeah Mega Altaria's gonna be fantastic, that typing allows it to check a plethora of mons and STAB Pixilate-boosted Return is such an easily spammable move. Dragon Dance/Return/Roost with the last slot having either EQ or Heal Bell is gonna be insanely hard to stop given the large amount of Pokemon that it can set up on and the limited number of Pokemon that can take an attack and KO back. 75/110/105 defenses are really really good for an offensive Pokemon - for comparison that's only slightly less overall bulk than Victini, but with an insanely good typing to go along with it.
Fortunately it's got a weakness to Rocks on initial switchin and is fairly slow even after a boost. But its typing is just so amazing, resisting the STAB attacks of EVERY common scarfer bar Jirachi. It'll be the best offensive Fairy, and it's even got a resistance to Fire which most Fairies would kill for. It can set up on, again, SO many highly used Pokemon it's unbelievably: Hydreigon, Mega Blastoise, Mega Ampharos, Mienshao, Suicune, Mega Absol, Shaymin, and Infernape are just a few of the strongest offensive threats it just says "fuck you" to.
TL;DR the combination of Mega Altaria's typing, boosted already-powerful STAB, access to Dragon Dance, and natural bulk is gonna make it close to broken imo.