The current porny fic I'm working on doesn't have a lot that makes it uniquely Potterversian, I recognize, but it has me realizing there's one particular mundane kink the Harry Potter 'verse has which other fandoms don't share so significantly:
Bondage with neckties.
Seriously, isn't that ever so Potterverse, that kink? Just imagine Harry's wrist bound with a length of green-and-silver, for example. Necktie bondage exists elsewhere, of course; it's not absolutely unique--but not with nearly as much significance as that schoolboy/color scheme thing does in Harry Potter fic, doncha think.