I think, that Mega-evolutions won't be as extensive as everyone thinks. They said that the first one will be an event thing. So, maybe all of them will be events, and each one will be designed for megaevolution. The coding will be in the game, but the pokemon won't be obtainable normally.
This mega stone, might be a one-shot item. They could just be, sort of like legendary versions of normal pokemon. Stronger, better, but rare, as they would be event exclusive. And like many other pokemon of that sort they might also be tournament banned. As well as in-game banned from battle facilities.
If it's like that, then this mega-evolution thing could work out well. Otherwise, it won't go well.
The only real problem I have with this, is Lucario's type change. Lucario has always been a dragon-slayer, due to the fact that it could learn powerful dragon and ice moves, AND was dragon resistant thanks to it's steel typing. Now it'll be weak to water, which is a LOT more common than fire. And it loses its poison immunity. And it's best ability, is inner focus, adaptability isn't all that great.