We already have regionals that aren't adaptations that happened over time actually:
Raichu evolves from Pikachu based on its diet, not Alola's environment. Its the local dishes that change its form. If a Pikachu in any other region was fed the same diet in other regions it'd still become Alolan Raichu, much like a Pikachu imported to Alola from other regions still can become Alolan Raichu
Exeggutor similarly evolves depending on hoe much sun Exeggcute gets before evolving. No matter where it comes from, if its exposed to Alola's harsher sun, it evolves into a new form
Marowak evolves as Cubone in Alola learn the local culture of fire dancers connecting with the deceased through their ritualistic dances. This is a change that happens because Cubone successfuly connects with its deceased mother through Alola's culture, not an adaptation of the species that happened over time.
The point being, we
already have forms that aren't the species taking a long time to adapt to the region, but rather being raised differently thanks to the region's condition, resulting in a different finak evo. The starter are likely the same.