I don't want to add to the shit-storm, but is there are certain people profiting from all the crimes and wars throughout the world, and they do it by preying on people's beliefs on religions, myths, fairy tales, urban legends, and general superstition. They predict/manufacture future events with it, they control people and events with it, they remain in power with it. So Batman and Wonder Woman have been teaming up over the years to discover more about them and stop their ploys.
The reason why i choose Batman is because i feel that Wonder Woman should be known more for her kindness, smartness, loveliness, and above all: her pacifism. So i feel using Batman as a juxtaposition works nicely.
For example i'd like to show that Wonder Woman can be just as smart as Batman. While he's very logical and have a wealth of knowledge, he's too stuck on doing things in certain ways, as if he was following a manual, - A and B follow C. Meanwhile Wonder Woman is able to sense things, things Batman would never pick up on, and come to conclusions in very unorthodox ways that goes against the conventional norm but it always click. So i'd write them as a pair of sleuth with two very different methodology.
Same goes somewhat to their methodology when it comes to violence. While Batman is human and has to rely on gadgets, he'd still always resort to violence when backed to a corner. Sometimes needlessly so for mere amusement. Meanwhile i'd have Wonder Woman be a ultra pacifist who despite her super-powers and militarized upbringing would always try to come up with a non-violent solution to any situation, no matter how dire. Because that's what i most like about Marston's Wonder Woman. She always came up with some crazy idea on the spot that didn't rely on punching to solve the obstacles. So Batman would be the violent one, and Wonder Woman the pacifist one.
That's my pitch. A globe-trotting mystery where superstition is being used as a tool of mass control.