I loved Supernatural. It was my favorite TV show and I watched every freaking episode.
In my honest opinion they shouldn't have done a prequel, they should have done something with Sam's Son and Dean's Daughter. Castiel could've gone to purgatory and cured Dean's Daughter from her monster side and teamed up with her younger cousin (Sam's son)
Lucy (the devil) could have somehow made a comeback and Sam's son and Dean's Daughter would team up to stop the evil
Also, Jack could ask Sam, Dean, and Bobby to go back to Earth with Castiel (now an archangel) since going against Lucy and company is no easy task.
The Winchesters are so boring. There's no passion in the series. Without Dean or Sam (main role) it's boring, absolutely boring. When I watch the Winchesters it's like I'm watching actors play a part, when I watched supernatural, it felt real and genuine.
Kill the winchester series and focus on the the future of Sam, Dean and family.