Does it need to be specific to fanfic? In the end, they're still just stories, sometimes shorts, sometimes chapters of longer works. What would you need that a general purpose publishing add-on could not provide? When I was doing up a plan for taking over a story and writing focussed site a few years ago, I was going to use @Bob's Article Management System. It plus Xenforo provided everything I needed, though I was looking at maybe adding a chatbox add-on as well. Plan was not needed in the end (the original owner decided to keep it and put it onto their new platform) but I would go the same route again if I was doing a writing site of any kind.