Since it's only the admins who can set up communities perhaps it would be a good idea to create a spot where we all can suggest and discuss what communties we are missing. For what it is worth it's kind of good that a themed/focused instance restrict community creation.

With that out of the way there are two communities I miss here and haven't found on the fediverse.

AskGameMasters - a place for gathering questions about GMing. And encourage them.

LookingForGroup - a place to post game adverts and perhaps also for players to post their desires.