Group Feature (Features)
I need it too. Every user can create his own sub forums(there is a max limit, maybe 5 per user), and set attributes like public or private, even add a icon for them. It should be a great thing! As admin only keeps the top level categories or second level categories, users will create and manage others. The owner of a sub forum will have some ways to manage the sub forum as an admin.
After all, only forums with high streams will need it.