It's been my experience and observation that fanfiction covers what canon leaves out. The more "slashy" a story is the more fiction it gets. The stories that have an actual pairing in them tend not to get as much fanfiction or the fanfiction is about non-canon pairings. I see a lot of Saiyuki fanfiction, but hardly any Kizuna or Level-C even though those are on nearly every yaoi fangirl's shelf. *takes a moment to eye book shelf* *happy place* AnK is an exception because the ending pissed off so many fans. The more popular fanfics will rewrite the ending and go from there. But that's yaoi fans. Slash fans might be totally different.

There's been a lot of fiction written based on Jane Austen's novels, because people love the settings so much. However, usually with a romance the romantic aspects are wrapped up at the end, so there's no need to embellish.