First we need to determine what fan fiction is, if its using the same world/mythology/characters as a previous work we'd have to toss out American Gods (fan fic of gods in modern setting), Good Omens (fan fic of Christian Apocalypse), the entire Percy Jackson collection, Röde Orm, and the Kalevala just to name some
Edit with a big example:
Der Ring des Nibelungen is a fan fic "musical" written by Wagner of the Nibelungenlied
I admit it was quite some time since I last watched it but I remember quite a few B's in the script