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There was a lot of real concern whether the SPN experiment would continue on after what seemed like a perfect capstone to everyone's work closed out S11, but CW renewed it again and this time Dabb stepped in.
The very beginning was very unusual as we followed two people, one investigating the Bunker left empty at the end of last season while another traveled the country, finding and talking with the protagonists of earlier seasons. We got to see a touching, yet funny moment where Dean & Lisa are sending Ben off to college, and Sam is going to drive Ben there since he's the only one qualified to give advice. We see Gamble's protagonists living happily with their second child and the tombstones of Carver's. When the storylines converge at the Bunker, we find out the one traveling about was actually Claire. And what we thought was one person in the Bunker was actually 2: Charlie and her friend.
There's some tension but things eventually settle down, especially when Castiel and Crowley come back. There's a very touching reunion scene between father and daughter. Charlie eventually leaves but not before dropping the bombshell that the Men of Letters had branches all over the world, and the British branch seems to have sent someone to the States.
There the standard MotW/protagonists-exploring episodes, then the show reveals that after dealing with the DarknessTM, Lucifer has taken over the body of a rock-star while Michael has taken as a vessel the Prime Minister of Britain (neither seems much interested in their fight any more). With Castiel's & Crowley's knowledge, they try to cram Lucifer back into the cage, but it seems Hell is locked up tighter than they thought. They improvise a kind of cage of their own and stick Lucifer in there then store it deep in the bunker.
They team up with the BMoL and - after giving him a chance to go back home and him taunting them about his own plan - toss Michael into an improvised container as well. The BMoL make overtures about establishing connections but Claire is the most vocal against it.
Back in the USA there's more conflict as the agent sent over there starts stirring things up, bringing the Princes of Hell out of retirement and getting some alpha monsters involved. There's back and forth, philosophical discussions about the way hunting should or shouldn't be. Eventually the other shoe drops: Claire is actually the BMoL agent with the free roaming one her "partner." (Though the show had provided red herrings that the other girl was the secret agent.) Turns out at the end of S4, the Novaks moved to England where Claire was recruited and trained for the previous 7-8 years. BUT then it turns out she's a double agent and turns on the BMoL and sabotages their efforts to consolidate power.
During this season we also learn that Michael's "back up" was that he had impregnated one of the Prime Minister's staffers and sent her over to America before the BMoL could do anything about the child. One of the Princes of Hell is after her because it wants to try and use the child to reopen Hell, while everyone else is wanting to get into the bunker and let out the archangels. Lots of stuff goes on, Cas and Crow argue over what should be done about the kid, Claire and friend only half-heartedly pick sides.
Things come to a head, shit hits the fan, Castiel and Crowley both die to save the day, and Jack is born.
Of course we can't go into too much detail here since that would inadvertently spoil our own S13. There is some tension between Jack and Claire as she had been warming up to Cas and feels like she lost her father all over again. The other protagonist does what they can to help Jack fit into the new world. Eventually he brings Cas back, first as a mortal, then restores the guy to a kind of new angel form that they spend a lot of episodes exploring. This allows the writers to give Castiel some abilities to help in plots without breaking them. There's also the further adventures of PoHs trying to reopen the place and uncovering what Michael had planned for Jack.
As for season 14? Well that would be telling.
But then again... this is all just hypothetical. ;)