I would be interested in doing a one-off based on this story - the one-off would have Sirius (and Harry) discovering the hidden chamber at #12 at the end of GoF (it would basically turn AU at this point - instead of Sirius having to flee, the Rat still winds up captured and Sirius is cleared, and Harry thus moves into #12 to live with Sirius, and they discover this room while cleaning #12 - complete with the original Narcy and Bella Black; Sirius smells Dead Fish, and, being the former Auror that he is, immediately calls Amelia Bones. The Order meets at Hoggy Warty - NOT #12 - and thus nobody except Sirius, Harry, Amelia, and possibly Croaker (DoM) knows about Narcy and Bella.)