Wizard x Cop OR Supernatural Creature x Cop
The serial killer is the talk of the whole city. He sneaks into secure homes and businesses, somehow overpowers his victims, then rapes and murders them in a grotesque fashion. The police are struggling to find a connection--until a man comes to then with information on the killer, and the fear that they will be next....

Basically, there is a black magic wizard who is raping and killing people to try and perform some big summons. The wizard-y community refuses to involve human authorities in the case, but when something strikes close to home a man decides that enough is enough. They make an 'anonymous' tip to the police, but after only a few hours there is a detective at their door demanding answers, and both suddenly find themselves stuck with unwilling partners.

Vampire x Window Washer
The really tall apartment buildings, the ones with very expensive pent house suites, often hire window washers to work overnight, so as not to ruin their tenant's view. One particular tenant is...a bit of a night owl. They begin to see the window washer every night, and a friendly relationship of scrawled notes and hand gestures provides more companionship then the vampire has had in years.