As I was writing the post, I considered public domain fandoms. I think it would be fine, but it comes down to (possibly) people using derivatives of the work in professional capacities, and then people writing fanfic off of those. Say Netflix comes up with an amazing series based on The Great Gatsby, which is in the public domain, and it inspires people to write fanfic. Sure, The Great Gatsby is in the public domain, but the Netflix production isn't. And if Netflix changes a character from the original George B. WIlson, mechanic to "Georgina B. Wilson, lesbian mechanic" then it becomes a really slippery slope if someone posts fanfic using The Great Gatsby, but they use Georgina in their fic, not George.

We need to discuss this more, including possibly ways to use GPL items safely.