Johnlock vampire AU... but with John as the vampire.
Seriously, the more I think about it the more fun it seems. Those cheekbones so sharp you could cut yourself, porcelain skin, great-brain-and-no-heart persona, the Byronic ideal personified, and he's not the undead one. It's just one trope after another waiting to be tweaked.
(And let's not even get started on the body-image thing, because why couldn't a vampire be a short, slightly stocky, ex-army surgeon gone a bit soft around the middle? How could that not be attractive?)
(There may be a bit of my offline life bleeding into that last one. I'm overweight myself, and someone at work is built like a bird but seems to still think she's fat, and is set on kneedling me about that kind of thing. So the idea of an immortal, devastatingly beautiful John Watson is... attractive just now.)