It's been 5 years since we last saw Adam who had the unfortunance of falling into the pit and possessed by Michael and currently remains there. It has been theorised that Adam has been tortured by Michael and Lucifer this whole time, but I don't think so as Micheal must know that if he ever gets out he needs a vessel of Winchester bloodline and Adam is the only living member of the family that said yes so he'd need Adam and he wouldn't exactly say yes again if he was being tortured.
Through a series of events some supernatural entity should gather the horsemen rings, Sam and Dean have 3 of them still and now that Death is dead I assume they will take possession of the last one. Now with the Darkness itself upon the Earth, a force that not even God could destroy, in desperation and as a last resort they could release Lucifer and Michael (possessing Adam) who in turn would revive Raphael and Gabriel would return very much alive.
Though all the Archangels, minus Gabriel, want to kill Sam and Dean they know they will all be destroyed if the Darkness remains and are forcedto relucantly team up as a family with the brothers which includes Michael and Lucifer. At the end Adam would either be killed, at least allowing him to return to Heaven rather than be stuck in Hell, or Michael could willingly release him for some reason allowing Adam to finally return to his life.
Just a thought but I really hope the producers find someway to bring Adam back and out of the cage, even if just for one episode, I mean he's a Winchester too and doesn't deserve what he got.