I could actually buy that as an alternative way of organizing texts -- instead of focusing on fanfic as riffs on copyrighted works and the others as riffs on the public domain (which is how it usually happens), you could use the term "fanfic" for those texts that don't seek to engage with an open audience, and have a separate term for derivative works that try to address people outside that pre-existing community. In other words, I disagree with you inasmuch as I chose R&G as a random example, and had I picked Scarlett instead, your point would not have stood (by your own assessment). But I think your point is a good one in a broader sense.

Still, when all's said and done, they're all works that involve playing in a sandbox someone else built. Which is the point I was trying to make.