I guess by now this is a lot bigger a question than just how to relate fan works to original works. We also don’t really have a way at the moment to say even “this is part of the Star Wars franchise” or anything like that. I’m honestly not sure there’s a good way to do that other than just tagging that won’t require a new entity type, though, and I’m not sure such an entity type makes sense right now.
I’d love to have some sort of transmedia franchise entity that can link to both MusicBrainz and BookBrainz though…