Okay, this was announced just a few days ago. We're going to be getting two more Pokemon with the power to Mega Evolve.

First up is Mega Beedrill.

With boosted Attack and Speed, and the ability Adaptability, it might be usable, but I'm not exactly enthusiastic about the design.

Next is Mega Pidgeot.

The increased Special Attack and having No Guard as its ability seems to indicate to me that it is meant for only one thing, and that is to spam Hurricane no matter what weather it's in. As for the issue of how Thunder and Zap Cannon won't have any trouble hitting it, in a Double Battle, if you send it out with a Pokemon that has Lightningrod as its ability...yeah. That's probably your best bet at getting around that issue.

Also, the Mega Latis purpose has finally been revealed. They're meant to fly you to islands that you wouldn't be able to access using normal methods like Surf. And on those islands, you'll find every single non-event Legendary Pokemon in the other games. That's the Legendary Birds, Mewtwo, the Legendary Beasts, Lugia, Ho-Oh, the Lake Trio, the Creation Trio, the Regis, Heatran, Cresselia, the Swords of Justice, the Kamis, and the Tao Trio. And there will be an Eon Ticket event so you can get both of the Eon Pokemon in the same game without trading.