I would love to see something where both Sherlock and Mycroft are vampires and they share John between them as their human/thrall/slave/etc.

Honestly, I can see this going in two different possible directions:
1) This could be utter crack with it being completely consensual with the brothers bickering over who gets John that night, fluffiness (as much fluffiness as you can get with vampires), etc.

OR

2) It could be super dark with John being an unwilling slave, rape/non-con, maybe abuse?

I would be happy with either direction, by the way.