Tbh I had seen this prequel idea on polls before over the 15 years of the show's run and honestly this was my worst nightmare come true, this was the prequel I didnt want, I was more open to a castiel prequel way more than this.
Personally and I know this is the unpopular choice but bloodlines I would have reinstated, I thought the idea was great and unique, heck even wayward sisters was better idea than this prequel.
I think that would have been it, in terms of other spin offs, I dont think a jack or crowley spin off would work, a part of me says an anthology like each episode sam and dean are in it but different versions of chucks multiverse, that could be interesting.
But what about you all, if you are fine with the john mary prequel that is fine, just personally Im not looking forward, I dont hate john and mary but they are kind of dull characters to me and prequels rarely seem to answer anything we dont already know.