What I would (have) like(d) to see explored more was the concept of the Day Watchers. No, they're not your Blade types. There is a feud between Light and Dark, like there always is, and as Night Watchers are agents of the Light meant to make sure that Dark abides by the rules of the treaty forged for the purposes of co-existance in the world, so are the Day Watchers to the Dark Side. They're Dark agents out to make sure the Light doesn't try to influence the choices of Others, when they become self-aware and must choose a Side. Although vampires as (some of the) agents of both the Light and the Dark was also a nice touch.
I actually liked Underworld, but this certainly made it seem like the plastic Happy Meal™ wrapped in vinyl that it was.