Frankly, I can’t think of a genre mash-up I want that isn’t already available.
Take any genre and you can add Comedy just by the way you play; you don’t need rules for that. Horror can also be interjected in just about any genre by just setting the proper mood. I’m sure opinions will vary wildly on this, but I usually don’t try to emulate a specific genre and be a real stickler that it NEVER devolves or evolves into something else. If we’re all having fun, I really don’t care if a sneaky, steal-the-MacGuffin adventure becomes a comedy heist.