I would be thrilled if people wrote fanfic about my characters, as long as they didn't make any money off of it. ;) I wouldn't mind in the least if I was selling millions of copies, though right now it would be weird.

I had people write to me who wanted to write sequels to my Tolkien fanfic. That I found slightly annoying, but I don't own the characters so they can certainly have at if they like.