I was thinking of a cross between Anne Rice's Vampires(Lestat)/Harry Potter's (Draco) /and Anita Blake's Vampires(Namely Jean-Claude and Asher). What do these gents have in common? In their respective fandoms(not canon), they are believed to be very sexy and very sure of themselves. I think it would be very funny to put them together and see its impact on the people who usually have to encounter them daily (although they may call it enduring torture. And yes, that is a challege. How could one harmoniously bring together two different vampire fandoms in one story?
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Another interesting one would be CSI (Las Vegas-I love Grisson) and Harry Potter.