To me, fanfiction is one of the greatest compliments an author can hope to receive. Fanfic doesn’t take anything away from either the original author or the original work, and it doesn’t harm the author’s vision in any way. How could it? Readers are never passive “recipients” of a story, anyway; they are always the ones who create the story in their minds through the act of reading. There are already as many different versions of the story as there are readers.
If someone finds an author’s fictional world and characters so compelling they want to pour their time, effort and creativity into spending more time within this world, engaging with it, and transforming it, then that cannot be anything but a good thing.
I read fanfic. I write it. I love it. And should someone create fanfic about one of my books, I’d be thrilled.