For example:
Tsuruya could have been a vampire in the series. (Because of the whole fang thing)

Her back-story would be that vampires are everywhere in the world but they burn and turn to ash in the sunlight so they have to live in secrecy from humans. The vampires suck human blood discretely every night to keep themselves alive but their victims don't turn into vampires unless the they conscienciously want to turn their victims into a vampires.

But three years ago vampires for some unknown reason were able to walk around in broad daylight without dying but their superhuman abilities would be suppressed during the daytime, allowing them to blend in more with human society. The vampires for some reason or another find out it was Haruhi's unintentional "Will" that gave vampires their day-walking abilities (Presumably to tear down the wall between "boring and normal" humans and "cool and exciting" vampires).

The idea is that Tsuruya was sent by the leading vampire clan to "guard" Haruhi from harm thinking that if Haruhi died vampires would be vulnerable to the sun again.