The ‘first’ rule of fan fiction is to not answer your own question in the body of the post, so that replies to it are all nested properly beneath for easy reading and sequestering, as is proper.

The second rule of fan fiction, is that well written fiction is harder to write romance fan fiction for than just writing an original character romance because you are by necessity changing the characters and their paths. They wouldn’t be the same character if they could have fallen for or gotten together with your particular pairing. This second rule naturally lends itself to thinking about…

The third rule of fan fiction, which is that fan fiction romancing is relatively easy for books that are poorly written or have severely underdeveloped characters. This can actually be used to determine if the book is anything other than bad. If the changed actions of the characters don’t make a fan of the series have a little wrinkle in their abdominal lining, then the characters don’t have strong enough actions and presences in the reader’s mind to create an idea of what a character might do in other situations for the reader.

The *FOURTH& rule of fan fiction, and my unpopular opinion, is that if your fan fiction was good enough to be read, it was good enough to have used original characters. Any popular fan fiction would have been better. Sometimes a setting is unique and fun to write in, or you can tell a particularly appropriate tale using the setting (harry potter and the methods of rationality come to mind, though honestly past the fourth chapter or so it wasn’t good anymore, just self-wanking), but most of the time the fan fiction author is simply being lazy and wants to use ‘pre-established’ characters.