What about Venonat and Venomoth getting new forms or a new evolution? They are the only Gen 1 Pokémon (apart from Mew who has been treated as an extra and a non-requirement for regional Dex completion) that has never been in a regional Dex outside of Kanto and Johto. Doduo and Dodrio were in Hoenn, Meowth has Alola and Johto, Mewtwo was in Kalos, and Pikachu has been in every regional Dex except Unova. The Venonat line deserves some love.
There are a few moth species in Hokkido such as the Hokkiaido Gypsy Moth and the Hokkaido Hawk Moth so its definitely plausible. The Hokkiaido Gypsy Moth is said to be able to cause widespread damage to trees and and can be an invasive species so Bug/Dark could work well.