I suggested this bunny in the comments of a Dean H/C Meme, but there's no telling if someone there will write it. This particular bunny doesn't want to stay and be friends, so he's hopping off into cyberspace to find another home. I hope someone adopts him because I'd love to see this fic become reality:
I wonder if the monsters in Purgatory have some sort of culture in place. The Leviathans probably ruled over purgatory, but now that they're on Earth, the infrastructure has collapsed, and the power vacuum has led to widespread chaos. Civil war has broken out between various factions of monsters, who are spread out among scattered camps and cities.
Dean is captured by one of these factions and treated horribly. Castiel or more friendly monsters (like Lenore) rescue him, but there's always the danger that he'll be captured again. Plus, any monster that met him on Earth knows that Sam will be trying to rescue Dean, so everyone wants to keep Dean close in case there's a chance they can escape when the portal is opened.
I imagine no one in Purgatory is happy with Castiel, but I'll leave that part of the challenge open.
Bonus points if Dick Roman shows up and tries to reclaim his throne, and Dean and Castiel have to stop him (either they're forced to do so or they have to confront Dick Roman in order to break out of Purgatory).