Yeah, at this point, I'm pretty sure it's Gen 1 only. At least right now. Though, if the feature is seen as a good thing (right now, everyone seems to love it), I'm sure that will expand in future games. Regional versions of Sinnoh Pokémon, or Sinnoh regional variants with a Gen IV remake? I mean, the Mt. Coronet evolutions were originally intended as Sinnoh variants, so I can see it working.
I am wondering what connection Alola has to Kanto, though, to have so many Pokémon grow there in different ways. Are we going to return to Kanto again in postgame?