Unfortunately, the idea that Dean's going to hell is part of the plan isn't too hard to believe when you're dealing with an angel. It's possible they needed Dean to go to hell and needed Sam to do whatever Sam did while he was gone. I do know that the show has always pointed to the idea that Sam is the one he's got to go after, but given who Dean is and his relationship with Sam...this is going to be good. Because the two of them would have to find some other way.