@grumpi I realise I'm quite late to this particular party.But I have started work on a solution:
It basically works - you need to grant the user access to the permission to create their own forums, currently done by making a request to
request has to come from an admin user. You'll then get a new button on the main navigation, which allows you to set up a new sub-forum, and designate who has access.
Obviously very much a WIP - but getting there.