The article below arose in the "wolf/kink" genre, hardly as widely read as, let us say, Alex Haley's Roots, which itself resulted in a celebrated plagiarism trial, substantial settlement, and Haley's "acknowledge[ment] and regrets that various materials from The African, by Harold Courlander, found their way into his book, Roots."
I think one paragraph summarizes the problem: << “In fan fiction, the sharing of tropes and story parts and plot lines is free flowing,” said Anne Jamison, a fanfic expert and associate professor of English at the University of Utah, who was skeptical of the notion that Omegaverse tropes could be copyrighted. “There’s a blurry line between what is specifically yours and what is somebody else’s.” >>
This notion has prompted reflections on my novel, Trojan Dialogues: The Memoirs of Diomedes. The work is set during the Trojan War. The Mycenean Greeks use Helen's "theft" by Paris as a pretext for initiating hostilities. Hector slays Patroklus; Akhilles slays Hector; a horse somehow contributes to the destruction of Troy. Those element are familiar, though certainly not "tropes." Moreover, my narrative is absolutely unique in the way so many plot lines are developed. Nevertheless, one might ask whether some contemporary author who had treated the same material a few years earlier -- e.g., Marion Zimmer Bradley (cf., The Firebrand) -- could have accused me of stealing her ideas. With the Trojan War, the answer is easy: of course not! With wolf-kink? We shall await the outcome.
The article is in many ways more legal than literary in nature, and the genre is one with which I am altogether unfamiliar. I must append that in classical music one finds a staggering number of variations written by one composer on the theme(s) of another. However, it remains to be seen how freely writers of fan fiction can continue to use the material of others.