There are Supernatural spin-off novels and comic books (and fan-fics) out there. However, I would love to watch two ideas to be made into a spin-off TV series.
I would love to watch a John Winchester spin-off. The ideal actor would be Jeff Dean Morgan for sure! Young Dean and Sam should be in it. Of course, young Dean should be by Ridge Canipe, and young Sam should be played by Colin Ford. There are other characters who I would love to see in this spin-off - characters who have only been mentioned and / or appeared once or twice in the original series - such as Caleb, Daniel Elkin, Pastor Jim and Bill Harvelle etc. They could bring in Ellen, young Jo and young Ash with a mullet as well! They could also bring in the psychic Massouri. Let's not forget Uncle Bobby played by Jim Beaver!! HOT!
Another idea for a spin-off series is Samuel Colt! I have always been fascinated by the Colt. Although, to me, it didn't finish off the way I would love to, but it is still a great idea for a spin-off. The ideal cast for Samuel Colt would be Jeremy Davis (aka Daniel Faraday from Lost) or Viggo Mortensen. Both are heavy-weight actors! It would be great to see a ghost-hunter on horseback hunting supernatural stuff and to see how he made the Colt - and how did he know so much; how and why he built the Gate and the giant devil traps (aka the railway line) etc. I think there are so much they can add to this one. Perhaps the writers could be build a connection to the Winchesters and the Campbells. That would be AWESOME!
WB! Eric Kripke! Sera Gamble!! Anyone! If you are reading this! Please contact me. I can write this for you! Or better... you go ahead and write it. I'd be happy to watch it!