[Formendacil]

At one point, I had a longer reply in mind, but those half-formed ideas dissipated long before I sat at a computer and now here we are.

I do have one thought, though, that sticks with me:

A good fanfic extends the runway when I've run out of the original and want something more.

That's probably not quite broad enough--a lot of fic deviates quite a bit from its origins (and I think Hui has a pretty workable theory), but I also find that I believe in Sturgeon's Law, and the 10% that rises above is going to do at least two things: tie me back as the reader to the original and give me something in addition to the original--i.e. in some way, add more, though "adding" could be changing/deviating/retelling/what-iffing as much as gap-filling or extending.