The Capacity to Love (http://archiveofourown.org/works/302659) Part 1 of the SPN Next Gen Verse

Three years after estranging themselves from Castiel, the Winchester brothers are still reeling from his betrayal, living a mostly-civilian life in Sioux Falls to protect Sam from further damage to his ever-weakening wall. But a new powerful force from Hell threatens to conquer both Heaven and Earth, and it's time for Team Free Will to save the world (again). Sam is willing to forgive and forget, but Dean still can't forgive Castiel for what happened at Purgatory's gate. But Sam, Dean, and Castiel must learn to trust each other again, because when Sam's wall finally goes down for good, Dean and Castiel have only two options: overcome their differences, or lose Sam forever.