I just thinked, if Mega evolutions are exclusive to Pokémon with their hidden ability? I mean, the event Mewtwo given away until September is based on the Mewtwo from M16, and the Mewtwo from M1 hasn't been seen transformed in the anime (correct me if I'm wrong, but I think Mewtwo for M16 isn't the same as the M1 one)... and that event Mewtwo has its hidden ability Unnerve. Plus this Torchic is given away with Speed Boost, and I think it is somewhat weird to see a Blaze Blaziken transforming into a form with Speed Boost when we know regular Blaziken can have Speed Boost as hidden ability (probably Mega Blaziken's changes will only be stats-wise). If it's true, Shiny Mega Pokémon will be difficult to get without RNG manipulation lol, or even impossible for some of them like Mewtwo, but I don't want this will be true.
I also thinked something else, more likely ; similarly to hidden abilities in gen V, it is likely than some Mega Pokémon are programmed in the game data, but will be impossible to get until their Mega Stone officially release, like Lightningrod Zapdos or Contrary Serperior who are currently unreleased. I imagine some Mega Stones available in-game, from a Dream World-like feature, or held by event Pokémon. For example, I don't imagine the Blazikenite being available in-game or from a Dream World-like feature and I think it will be exclusive to this Torchic event, and I don't think gen VI Pokémon will release with their Mega Form for a while, considering gen V Pokémon with their hidden abilities where unavailable prior to BW2 outside of Zen Mode Darmanitan and Telepathy Musharna from BW1 in-game. Talking about the Speed Boost Torchic event, I hope it will be able to be female, so Speed Boost Blaziken will not be an event-only thing anymore... oh wait, Baton Pass Speed Boost Blaziken. That will be broken.
I wonder how Mega Stones will work in-battle. Maybe they will activate only when the Pokémon takes a hit? Or automatically change their form in battle if they hold their proper Mega Stone when they come into play, or after a certain number of turns? Or each Mega Stone will have its own activation conditions?
And I know how exactly will lock Mega Charizard :
http://1-media-cdn.foolz.us/ffuuka/board/vp/image/1364/23/1364230913195.png (I'm actually joking, of course)