One maybe an alternate beginning to season 4 when Dean comes back from hell, maybe he's a lot more traumatized, doesn't remember who he is, or Sam, and he needs Castiel's help getting reacquainted with the world and the 'job' the angels have for Dean. During that time maybe, because he's so confused, the feelings that Dean had for Sam get transferred to Castiel since he's all Dean has right now and their relationship evolves into something neither of them expected.
Then maybe it could go one of two ways, either Dean starts to remember who he is and wants to leave Castiel and go back to Sam but by now the angel doesn't want to let him go. Or they somehow cross paths with Sam and Dean still doesn't remember who Sam is and it becomes a tug of war between Sam trying to get Dean to remember him and Castiel wanting to keep Dean for himself.
Just rough ideas, would love to hear anyone elses ideas.
