I would love a city kit. You could put in some skyscrapers, apartment buildings, subway, trolley, cars, shopping, roads, busses, street lamps, night/day city elements, etc. Some grunge/rough textured papers, modern looking steel type photo frames and elements. Some text adjectives to describe city life (for residents and vacationers) like busy, modern, trendy, etc. Certainly black as a color, but other city type colors -like brick/maroon or park green. Maybe if you end up doing the city at night you could throw in some jewel tones of purple and fushia, neon blues, something bright like city night colors.
I could certainly use this kit. I have Lauren Grier's In the City, but some of the colors don't fit my daytime pictures I have of New York or Boston, nor do they work at night (in some areas). I bet others have lots of pictures of going out with friends at night, visiting cities, etc.
Another idea might be City Themes -like Paris, or London, or New York, since these are visited often and have unique characteristics.