A thought has been stewing about inside within the confines of my cranium for about a fortnight, now; at the moment, it's only a innocent, incomplete observation, mixed with a bit of imaginative speculation. In no way is it designed to be a serious prediction, but I thought I would share it for antiquitie's sake: It's far too early to say now, and as of late no one has been capable of accurately predicting Game Freak's design direction for the most part. However, if I were forced at the threat of grievous harm to make a gut decision in regards to the possibility of mega evolving Pokemon recieving an alolan form, or creatures in possession of the newly revealed form divergencies eventually mega evolving, I'd say it's not likely to happen. That would mean no Alolan Charizard, Lucario, Salamence, and so on.
The alolan forms are a rather new concept, and regardless of the magnitude of it's impact in Sun and Moon, there is little doubt that future installments shall likely harbor similar, region specific alterations to older pokemon. So, it's rather difficult at this point to envision precisely in regards to this issue, as the Pokémon Company could, any day now, reveal an Alolan Charizard, or a mega evolution of Exeggutor (as unlikely as they sound at this point). Still, I can't shake the feeling that it is quite possible for each and every Pokémon currently participating in either category to stay exclusive to one or the other. There is currently nothing concrete to suggest this; only the mildly important, and possibly coincidental, observation of noting that no mega evolving Pokemon has received an Alolan form as of yet.
The main reason behind this sudden conjecture of mine, is simply imagining the theoretical awkwardness of the same Pokémon possessing both attributes, while avoiding the realm of confusion for the game's participants. I've asked before, on these very forums, just how an older pokemon, say a cubone, will react when transferred to Sun and Moon. In the trailer, it demonstrated that these Alolan Forms are achieved through evolution; hence, when it evolves, which Marowak shall it become? Many have suggested that a natively captured cubone will turn Alolan, while an imported Cubone shall evolve normally. This would be achieved, theoretically, through some sort of code separating the two variations.
Moving right along then, let's say they give Marowak a mega evolution in a future title. The regular Marowak, that is; it turns Ground/???. The alolan Marowak, logically, should be unable to mega evolve in such a manner, seeing as how it is not only ghost and fire, but thematically different as well. So, now you've got two sets of cubones, and while one evolves into a ground type who mega evolves, the other, caught only in Alola (yet identical in every other way), evolves into a ghost/fire type, and cannot mega evolve. Would it then require it's own mega evolution? Well, that would depend on exactly how much these processes are to make logical sense in junction with one another. Similarly, let's say Blastoise receives an Alolan form; perhaps it becomes fire, grass, or what have you. Would it still logically be able to mega evolve into a massive water turtle with a hydro cannon on it's back?
So, I suppose I chalk all of this speculation up to logic and implementation, which I at least believe that Game Freak attempts to monitor for world atmosphere and quality control. Maybe having the two separate coded Cubone's wouldn't be that big of a deal; once we leave Alola, I suppose we'd be back to catching the regular cubone, and it'd be ripe for Mega Evolution. Still, worrying about keeping them separate and identified, for trading (especially among younger players) would still be a concern no matter the situation.
I bring this up centrally because many are discussing whether or not the Gen 1 starters, the gen 2 starters, or most commonly the gen 4 starters, could receive an Alolan form change. Well, for one, it appears as though it's possible that the only generation to possibly receive them is the 1st. Like my earlier "mega evolvers have not received any Alolan forms" statement, this too could be mere coincidence. Still, with what I have laid out as conjecturally permissible, it's a decent possibility of Charizard, Blastoise, and Venusaur (as well as all of the other, many mega evolvers) being left out in the regional form alterations. Also, because of the seemingly obvious design choice of eventually mega evolving all starter pokemon, including those from Johto and Sinnoh, I suspect that they, too, will not receive regional variations.
Still, this is all convenient speculation based on what we know now, and what has erected itself through chaotic theory. It's far too early to adopt this sort of mindset now, and there's certainly no need to worry one way or the other. None the less, I believe that there is certainly some scruptious food for thought in what I have pseudo-obnoxiously posted here, wall upon wall, before your precious vision receptacles.
Edit, Duke of Eddington: Oh, dear; perhaps, may I additionally observe, that I have grievously posted this tripe in the wrong forum category! Gadzooks!!!