Just before the finale here is what I thought:
Sam would go into the pit with Lucifer, but Michael would get locked in there too. While they were in the pit Michael would be cut off from Heaven, Lucifer from Hell and Sam from humanity. At first, no one spoke, and that lasted a really long time (like decades). Sam found a way to communicate with Jesse, but it was unreliable and Jesse only understood a tiny bit. Then finally the angels started to talk to each other. After 400 years the angels reconciled and Sam Lucifer and Mike all headed back to Heaven.
Sam hung around heaven for awhile (100 years) but when he was given the chance to return to earth and human form he took it.
He arrived on Earth somewhat angel like and confused. His behavior was scary after years with only L and M for company. He had many of their mannerisms and also strangely, some of their grace and power.
Jesse warned Dean that Sam would return to ease Dean into the idea. It’s been 5 Earth years. Dean is stable now, with Lisa and Ben and also caring for Jesse. Dean wants to protect his family as much as he wants to see Sam. It’s a shaky reunion, but slowly, Sam regains his sense of humanity and human behaviors.
Eventually the boys are called to action and go off on a mission together.
Sam before Detroit. He's so scared for what he's about to do, and the consequences if he fails.
Lucifer glows and begins the transfer to his perfect vessel.
Something takes hold of Lucifer, and all the memories of brotherly love flood his mind.
Sam has regained control of his mind and body. Love beats anger! Free will trumps destiny!
Sam surrenders to his sacrifice with perfect humility.