@grumpi I realise I'm quite late to this particular party.But I have started work on a solution:

GitHub - mileswilson/nodebb-plugin-private-forum

Contribute to mileswilson/nodebb-plugin-private-forum development by creating an account on GitHub.

GitHub (github.com)

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

/api/promote_to_gold?user_id=<required_user_id>

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.