There are two (technically three, four if you count Okinawa) major regions of Japan left to cover. With international travel for research being difficult the last two years, a new Japanese region is very possible. I'd be for it- previous Japanese regions don't really lean in to the culture as much as Gen 5-8 did with their real world basis. Johto kind of does, especially in HGSS, but doesn't really make up for the other three. I mean, we've still never seen a Japanese castle!
Regional forms could be mostly gen 5-8 Pokemon too, which would be a fun contrast to how most regional forms before we're of early gen Pokemon.
