One of the things that bothers me about Game Freak basically abandoning old gimmicks and going for new ones generation after generation is that they don't really give the time to properly flesh out the original gimmick that they had in mind before ditching it, mega-evolution included.
In Gen IV, while a handful of Pokemon got cross-gen evolutions, this was never really expanded upon because there are still plenty of other Pokemon (mostly Johto) that would've appreciated an evolution. The same applies here with mega evos -- there are plenty of Pokemon that still deserve of a mega evo of some sort -- but just haven't gotten an opportunity for that, and it seems as if Game Freak just kinda tossed that idea to the side never to be fully fleshed out again once a new generation is on the horizon.
It's frustrating, really.