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 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.
Just want to plug !pf2general@pathfinder.social and !pf1general@pathfinder.social for PF chats, it’s on a different instance but we’re around!