What if Cas get out from Purgatory much much later. Unaware of that he would go to Winchesters only to find that he was gone for 50 years. Dean is dead already and Sam is living alone, old and broken, waiting for his own end.
But he still smiles brightly when he sees Cas and says "I always hoped you'll come back to us, Cas." and with this smile he suddenly looks so much younger, wrinkles arranging in dimples, like in good old days.
He and Cas goes to Dean's grave and Sam is rambling to his brother about Cas' return and that he's happy and he knows that Dean would be happy too. Castiel is just standing there, regret and pain too intense to let him speak. But Sam takes his hand and places it over Dean's gravestone ("He was cremated but I still wanted to have a place to visit him") and then holds him in his old, fragile arms when Castiel cries.
Cas wants to take Sam to a trip all over the world to show him all amazing things he hasn't seen yet. Sam just shakes his head and explains "I'm too old for this, Cas. You're here, Dean's here, why should I go anywhere?"
So they stay in Sam's house, Cas watching over his friend, every day. He's with him when he's growing weaker and weaker, but still smiling fondly at the angel when he catches his worried look. He's with him when he finally reaches the end of his long, meaningful life.
The last words that Sam says to Cas are "Thank you".
