Right, but still, there is only so much fanfiction produced for any given franchise, as there are only so many fanfiction writers able and inclined to do the work.

Now imagine if anyone could casually just fork a piece of writing and adapt to their personal tastes. "Like this, but make X be in love with Y instead of Z. Also make Z not really die but make a surprise comeback in the final chapter." It might be fun to play with such capability for a moment or two, but I can't imagine anyone enjoying exchanging such stories - if everyone can trivially change anything, then everything becomes arbitrary. There is no story, only everyone looking at their own selfie.

Incidentally, GPT-3.5 and GPT-4 already give us this capability with smaller pieces of fiction. You can literally alter any short story like I described above, today. Anyone can. And somehow, I don't see any takers. I think people realize there's no entertainment value in it. Hence, this is what might just save Hollywood et al.

Erotica is one exception to what I described above, as it's by nature the kind of fiction to be enjoyed personally, not in a wider context. And, of course, erotic story generators are as old as the Internet.