I'm not sure how the show will end, but I think I know where it's going. We are cruising towards another version of the last half of Season 5 IMHO. Think about it, we have Cas human, the angels have fallen so they will soon be out of power and out of the picture. We have an angel inside Sam again, what if he has ulterior motives? What if he was friends with Lucifer before he fell and is loyal to him? What if Ezekiel breaks Lucifer out of his cage and gives up Sam as host, then whalla we are basically at the bleak future that Dean saw in his trip to The End.
Maybe fate and destiny are inevitable?
All IMHO of course, I honestly have no factual idea where this is all going.