So fans have fan fiction because they love certain worlds and characters and want more of it, or to riff on the base to take stories in new directions. We have things like TV tropes so it is no small stretch to, say, generate sitcom-type stories from Harry Potter or GoT that can continue forever. But where does all of this lead? Music has sinilar issues, but I think of this like the compression/loudness issue in musi…
I'm not sure whether Harry Potter pioneered the "school + wizards" mashup, but the school has many aspects you would expect of a school (save for the wizards) and the wizards have many aspects familiar from previous fictional wizards (save for the school part.) The novelty comes from combining the concepts in more or less straightforward ways.
Even a writing AI that is only capable of taking an existing plot and adapting it for a different setting could produce enough reading material to last a lifetime by taking all existing novels in the public domain and putting each in a different random mashup for modern audiences. And I think many people wouldn't tire of this at all.