Downfall of Supernatural Spinoffs Began Long Before The Winchesters
If you don't like the John and Mary prequel, remember one thing: at least it's not Bloodlines.
OG Supernatural fans remember very well that The Winchesters is not the first attempt to launch a spinoff of a massively popular monster-hunting series. However, the first attempt was far from successful.
Back when Supernatural was in its ninth season, its 20th episode was supposed to be a pilot for a new spinoff called Bloodlines, which would explore the "clashing hunter and monster cultures in Chicago." The backdoor pilot episode followed Sam and Dean as they visited Chicago and discovered that the city was torn between powerful monster clans.
Things did not go quite as planned. Currently, Bloodlines remains a single episode, which is also Supernatural's worst when it comes to IMDb ratings, as it has 5.8 out of 10 on the site.
Bloodlines had several problems; not only did it have a weak storyline, but it also came on the heels of pretty compelling previous episodes of Supernatural season 9, which introduced the angel Metatron as the main villain, but never forgot to deliver good "filler" stories. Besides, the enormous amount of clichés and a general parody vibe of Bloodlines made sure that it was dead on arrival.
The main problem with potential Supernatural spin-offs, according to fans, was that the universe actually relied heavily on the presence of Sam and Dean. Without the two brothers, Bloodlines could only offer a generic dark fantasy plot.
Even The Winchesters, which is not technically about the brothers, still only keeps the fanbase hooked by teasing occasional appearances by Dean Winchester and sticking to his narration.
Some fans believe that a spinoff focusing on Castiel or Crowley (or both) may be the first exception. However, there are currently no official plans for such a project.
The Winchesters returns on The CW on February 21.