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.
There is a theory as to why they are Kalos only.
Sycamore explains his theory of how the Mega Stones came to be. During the great war when AZ fired off his doomsday weapon, the resulting energy corrupted the area, such as creating the ainstar sundial. The energy from the sundial was the same as the energy emitted by the Mega Stones, so he believes that they were created the same manner.
So the mega stones were created by the energy of the weapon and since the weapon was only fired in Kalos, they'd be around Kalos.
So while the Mega stones are native to Kalos, they can be given out in other regions. Like with HG/SS however where the Evolutionary items that weren't originally native to Johto despite there being many pokemon that could evolve with them in the Johto Pokedex, I agree that mega Stones will not play a major role in the ORAS story. They most likely will not be available until Post game to which we'll be given them or probably can buy them.
Well, actually, didn't Red obtain a Charizardite X in Lavender Town? If they could retcon Kanto to have mega evolutions in Origins, I'm sure they could happen in Hoenn as well. Masuda also said in an interview that Gamefreak will be monitoring VGC competitions to get ideas for the future of megas.
Red received the Charizardite X from Mr. Fuji who was given it while he was traveling in his younger days. Mr. Fuji didn't get it in Kanto so even though Red got it in Lavender Town, the Mega stone wasn't from Kanto.
If they were to do something similar in ORAS, there's a high chance that this won't be until Post game since the Mega Stones weren't in the original games and remakes stay parallel to the original games. Even HG/SS only included the evolutions that weren't optional. Only new Pokemon that were added to the HG/SS Pokedex were the ones that evolved after learning a specific move that was natural to those Pokemon.
For Pokemon that were location based, you had to trade those away to a game with those locations, which would again pass on into these remakes so even though Nospass is native to Hoenn, he can't evolve unless we trade him to XY and evolve him there and send him back.
Mega Stones are part of the main series, like with evolutions, they'll reappear but it doesn't have to be during the main story. There is a good chance that they won't appear at all during the main story and only appear as something to collect Post game.
