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.
Just want to plug !pf2general@pathfinder.social and !pf1general@pathfinder.social for PF chats, it’s on a different instance but we’re around!
Just echoing mostly, but…
DM/GM academy (which is right in line with your first one), homebrew, LFG (again, just like you said) and probably worldbuilding should be made sooner rather than later.