>>34347599The Alolan forms were created specifically for the 20th anniversary. It is unlikely we will get any more regional variants until we hit yet another anniversary milestone and/or Gen VII remakes. Hell, the fact that we didn't get any new Alolan forms for USUM supports them just being 20th anniversary even further.
But then again Game Freak changed their minds about not releasing Pokemon games on VC, as well as limiting customization options to just in XY, so maybe we might get some regional variants in future games if we keep pestering them.
The short answer is: Don't expect any new regional variants for Gen 8 until we pester the shit out of Game Freak just as we did for ORAS.