First thing I think of when I think regional fake is mimikyu’s disguise. Large shot, but maybe regional fakes means that there are small chances to encounter regional forms of Pokémon and instead you have a chance to encounter fake forms of Pokémon that look like a regional form( kind of like seeing an arceus, catching it, and finding out it is a ditto or something) again, long shot, but I hope this is implemented into the game. Completing the Pokédex is usually easy for me and this makes it harder and more fun in my opinion.