Except I don't want it to take place in LA because I don't know anything about LA.
Story Seed time! First, some background information on the concept; but if you want to get right to the idea, you can go down to the jump.
Between Heaven and Hell, there is Earth. And it is upon Earth that the greatest beings walk - Mortals, complete with undying, unending souls, energy that powers life itself. This energy in all things, makes Earth rife with power and thriving with those trying to get a chunk of that power for themselves. That being said, Angels and Demons, the inhabitants of Heaven and Hell, go to Earth for one of two reasons.
1. To become mortals, and live. Angels and Demons are imperfect, compared to Humans, in that Humanity can truly and freely pick it's own options and make it's own destiny. Nothing controls a human if they don't want to be controlled, wheras Angels and Demons are ruled over by hierarchies of power.
2. To remains as Angels and Demons, in order to do what they do best.
For Angels, that means inspiring, loving, growing, tending, saving, blah blah blah all that fun stuff. Although Angels aren't all "good" by our terms. Angels are also present in warfare and death.Ultimately, Angels are worried about the other person. They are creatures of a greater good, and they seek to preserve life in all it's entirety. Angels are more or less locked in the amount of members there are. More Angels can be made out of love and dreams and whatever, but more often a new Angel is simply generated whenever another angel is taken out of the picture.
For Demons, that means stealing, seducing, and controlling. You get the picture, and like Angels, Demons are not all "bad" by our terms. They can also be charitable and kind, provided that there's a payout for them in the end. Demons are ultimately self-centered and a little bit pitiable in their greed. Demons breed and spawn and produce more and more of their members all the time, often using humans as breeding stock or even tricking humans into raising their spawn like cuckoo birds. Fortunately, they also have a high enough turnover rate from their violent nature that their population tends to stabilize.
Earth, though, is a middle ground. Despite what most think, Heaven and Hell cannot see everything upon Earth, far from it in fact. So Heaven and Hell's upper management have to rely upon what they know from Angels and Demons and other such things for the latest news.
Of course, "There are more things in Heaven and Earth than are dreamt of in your philosophy" so Angels and Demons aren't the only things wandering around Earth. Earth has it's own host of creatures that don't necessarily belong to heaven or hell (although some of them can wander to one place or the other on their own, or to even stranger places) including, and not limited to, therianthropes of all sorts, vampires, monsters, ghosts, and more or less your standard Monster Squad crew.
And now, under the cut, the actual Story Seed.
Angels leaving to Earth to become mortal is no strange thing, but when a woman who was formerly a very powerful Angel is murdered, a rather young and naive Angel (who was generated when the more powerful angel left), somewhat confused and disturbed by the concept of a death, takes it upon herself to find the reason behind the woman's death, traveling to Earth.
Meanwhile, on Earth, a Detective (who is also an atheist, as his wife's(?) death destroyed his faith) is investigating the murder of an artistic, well-loved woman (our murdered Angel). When the evidence doesn't match up, he does some extra digging, and finds himself on the wrong side of group of vampires hired to clean up anything that could lead back to the murderer.
After a brief scuffle in the dead woman's apartment, a being of light and beauty saves him.
From there on, it becomes a buddy cop movie involving Demons, Angels, a plot to destroy Heaven, vampires stealing angelic blood to become mortal again, and shenanigans.