I like this idea as well, and I don't mind helping if possible
Although tons of people have probably already said this, I had an idea like this too, except that each city would have a town center where they could capture it if necessary, and different factions of the Earth would rival to create countries.
The trick is, however, to get everything in proportion. Perhaps from the last level in New York to the first level in Washington could be 2-3 levels apart, maybe even more. Then from Toronto to Calgary it would be much more, maybe 15-20 levels.
There will be lots of empty space on the overworld, but that's ok.