That is an interesting theory actually. However, I have a feeling that Mega Evolutions won't be making an appearance in Hoenn.
I do recall that the Kalos region is the home to Mega Evolutions, and the Mega Ring. I could be wrong, but I think that Kalos is the only region we'll see with Mega Evolutions until generation 7 at least.
Seeing Mega Evolution in Hoenn wouldn't make a lot of sense either. These are remakes of the original games, and given that the discovery of the Mega Stones in Kalos is such a major deal; it would be really odd to see them in Hoenn since there was absolutely no mention of them in R/S/E. The adventures in Hoenn are also supposed to be set around the same time as Red's adventure in Kanto. This would most likely place X&Y and Kalos further into the future. To me, the remakes will probably be based a few years before Mega Evolution was publicised, and thus I don't think that there will be much involvement of it in Hoenn.
There's also the fact that there were no other Mega-Evolutions programmed into X&Y's data that we haven't discovered. So I'm going to rule out the possibility of Steven giving you a Mega Stone that can be traded back to X&Y. So I'm deducing that there won't be any more Mega Evolutions this generation given that it would make no sense for any to be in OR&AS, and there aren't any more in X&Y. That said, I believe that Steven would have "heard of" these new Mega Stones, and perhaps he's going to investigate them. I just doubt that there's going to be any new evolutions. New evolutions would also shake up the meta-game once again, and that's something GameFreak and Nintendo do twice a generation. Nor would it be wise to do so.