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.
Actually, we're only set for admin-created communities for a short amount of time; After a few days, i plan to turn that off and let anyone create communities, like on reddit. The reason for the restriction is to prevent someone from rushing here and "squatting" on a community that might want to move here from reddit, to get a hold of it before the mods there do.
That is one good way to go about it. Letting the dust settle and all that. Still, if there are folks out there with good enough IT access willing to set things up and do the work I would love for someone else to take the initiative. Sounds a bit selfish but I only have decent IT access during the evenings so I cannot take on main responsibilities.
Keep up the good work and to all the mod inclined individuals out there - wink-wink hint-hint
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.