So let's say that Purgatory is actually a part of the Nevernever. Thus, Dean and Cas not only have to deal with monsters, they have to deal with wyldfae and the Winter and Summer Courts. And word gets around about the Winchesters and their angel, so the moment someone hears that one of the Winchester boys and the angel is in town, it's pretty Dean and Castiel Season in Purgatory.
Meanwhile, Sam? Is busy looking for a way to get Dean and Cas out of Purgatory and deal with the Leviathan problem. Unfortunately, he's not having much success, so he turns to the only wizard in the phonebook for help (or, if post-Ghost Story, the Winter Knight). He's expecting a fearsome, scary, no-nonsense man to show up, judging from all the talk about Harry Dresden, but instead gets the wise-cracking, pop-cultured, chivalrously perverted wizard that somehow reminds him of both himself and Dean instead.
So basically? Dean and Cas surviving in Purgatory which happens to be a part of the Nevernever, while Sam and Harry are working to bring down the Leviathans and get them out of there. Pairings are up to you, but I can never resist Harry/Sam, since there's so little of it.
Dresden Files crossover
Meanwhile, Sam? Is busy looking for a way to get Dean and Cas out of Purgatory and deal with the Leviathan problem. Unfortunately, he's not having much success, so he turns to the only wizard in the phonebook for help (or, if post-Ghost Story, the Winter Knight). He's expecting a fearsome, scary, no-nonsense man to show up, judging from all the talk about Harry Dresden, but instead gets the wise-cracking, pop-cultured, chivalrously perverted wizard that somehow reminds him of both himself and Dean instead.
So basically? Dean and Cas surviving in Purgatory which happens to be a part of the Nevernever, while Sam and Harry are working to bring down the Leviathans and get them out of there. Pairings are up to you, but I can never resist Harry/Sam, since there's so little of it.