I am interested in this discussion as well and would be into exploring the use cases more. It seems to me that each discourse forum should be independent and separate. But maybe not right away. At first as it is being evaluated and new forum owners and members learn the ropes it would be hosted as a group on a forum run by someone else for them, like wordpress.com for new bloggers. Eventually as the group grows and takes wings, and needs grow, it could be moved to its own independent forum.
How this would work I am not sure, but the ability to export part of a forum, Eg a group and all related users, categories, and topics, would do it.