What I really don't understand about generative AI, is why would you want something else to write fanfic FOR you?
You feed a human a prompt; and they'll give you something full of passion.
You give the same prompt to AI, and it just plucks from everything that's already out there, with no thought to "oh, I love this concept!", it steals, but it doesn't even do that with a passion.
And if you want something extremely specific, if that's your justification, then you'll have to give so many keywords and phrases, that you might as well just write it yourself, and get exactly what you want