I think the problem here would be how does one define a City?
Countries, States, Regions etc are all defined politically in a way that is fairly easily recognisable across the world, but cities are far more fluid. In New Zealand up to 1989 a place only had to have 20,000 people to be a city. Now it's 50,000. Other countries with much higher population densities would have much higher qualifiers than NZ. Some Cities in the UK are classed as such because they have a Cathedral in them.
Too many variables that would require constant monitoring for changes to be viable.