I haven't made much progress with the new forum yet, but I've worked out how to manage permissions.
At the bottom level will be users.
Users will be in groups.
Groups will have permissions in particular forums (so a user can be a forum moderator in a particular forum, but a simple user in another).
So permissions are basically worked out (writing the API will be a little bit of work but shouldn't be too difficult now I've worked out the logic behind it all).
Then, forums will (potentially) have subforums.
A forum will have many threads.