This one's at the zygotic phase right now so there's little to report except for the news that The CW is planning to extend the Supernatural franchise well beyond its forthcoming ninth season, according to executive producer Bob Singer.
Returning for season nine this autumn, Supernatural is set to introduce a new character around episode twenty of the ninth run, who will then form the basis of the spin-off. The planted pilot strategy was used for the network's The Vampire Diaries spin-off, The Originals, starting later this year.
TV Line report that the new show is to have a "citified feel" in comparison with the Winchesters' rural demon-hunting backdrops. Less of a Batcave than a swish converted loft perhaps?