Plus, I think every new generation with remakes has written things off more as "oh, new Pokemon are appearing in [Region]!"
So if we returned to Kanto, or something, then either post-game or just through the course of the story, Alolan Pokemon - or even a mechanic like Kanto Forms - could just appear wherever, or in a new location. Maybe the Sevii Islands.
I think it's pretty cool to note that Kanto and Alola seem relatively close to each other from in-game dialogue, so that could be a major factor in Alolan Forms only applying to Kanto species thus far. If the regional forms mechanic continues to be a part of the series during this generation, much like Mega Evolution in the last, then it may be reasonable to expect a similar pattern of geographic proximity. If we stay in Alola, then we might see some Johto species or Unova species, since Johto is right next to Kanto and Unova is the next-closest based on their real-life origins. If we were to move to, say, Kanto, we might see Johto forms; if we moved to Sinnoh like many suspect, then that also might give us some more Kanto species or Johto species.
Honestly it would be cool if this were less like Mega Evolution, in that they might consider creating at least a few new regional variants even in future generations rather than letting the new forms stop coming.