Logically, I know that there's fan fiction (general, slash, dark, het, 'shipping, whatever) for everything.
I mean, I once read a fan fiction that was a cross-over between Dragon Ball Z and Anne Frank. Yeah, the Jewish girl who hid in an attic with her family as well as others.
Still, never thought I'd find The Dark is Rising slash.
I do find it interesting that the authors tend to make Bran the more... aggressive one (in the way that he's the one who does the pursuing and seduction and such) and not Will. The other way around makes more sense to me, but what do I know? I never before imagined the two of them interested in anyone in a sexual way.