Okay so El Chico Eevee has made two posts talking about Mewtwo x/y getting changes to make them more like zachian. And a post about mega Charizard x potentially coming to the game. So my theory on what's going to happen is there going to add mega stones as held items to trigger mega evolution. I believe this is the case because El Chico Evie states that both mewtwo's are getting a change that will make them similar to zachian. That change is potentially only having two item slots because one item is forced upon them. I believe this item will be a mega stone. How this relates to mega Charizard x is because apparently there will be a new holloware that would apply to both base Charizard and mega Charizard. If they were two separate Pokemon within the game why make one holloware that applies to both of them. I think what they're going to end up doing is adding megas to Pokemon that are already in the game and the way you trigger them is by having that particular Pokemon hold a mega stone. So Charizard would have to have a mega stone in order for it to become mega Charizard x within a match. And the reason this would be forced upon Mewtwo x and y is because their entire gimmick in the game is already based on their mega evolutions. Unless they completely change the way their mega evolution works it would make more sense for them just to be forced to hold the mega stone to begin with. 

My theory on what these two new features mean for unite