My only idea is Matt Cohen and two young J2 clones as it'd be the only alternate version of Supernatural I would tune in for. Also by J2 clones, I do mean clones, down to the chemistry. Never going to happen so I won't be watching whatever spinoff they come up with. They can barely manage the main show as it is lol.
I had several scenario's but after you originally posted it I think this is the best option. They have already had a great young Sam that was on for several years and they got a really good young Dean in about a boy. If they could find two new versions the show could work. As an added bonus they could work in a pilot episode or some spin off eps into the season. J2 would only have to work a day to bookend it and the story would still be all about Sam and Dean.
Adam Glass once joked that Supernatural should be like Dr Who with a new pair of Sam and Dean every few seasons. I didn't care for the idea back then because I didn't like the idea of anyone replacing Jared and Jensen and I still don't want other people pretending to be them but I'd love a pre-quel with Young Sam and Dean that would end with "Pilot". Full circle lol.
Exactly, and if they started them young enough it could go on for 10 years. But if it was cancelled early and ended on a cliffhanger it could be resolved in the main show.