I don't think they should make a sequel, I think they should just crank out half a dozen trailers like the one for the first movie. That trailer was just so badassed, and so much more enjoyable than the actual movie. Now, I'm not one to give any respect to those fan-made superhero trailers, but since the Underworld makers concentrated 100% on style over substance, they should just condense that substance.
There's no business model for it but, hey...