I forgot to add brutti_ma_buoni's "Rulesverse" which is quite elaborate (in a good way) and covers the post-series establishment of the Slayers Council but focuses on Giles/Faith as the main 'ship. But it also has fun with organizational constructs, and the story where Giles overhears IIlyria acting as a "guidance counselor" to one of the new Slayers is very funny.