If they add cities, they should add 4.

Place them slightly outside the edge of the currently available map, and use the new space to add more natural shape to the game map so its not perfectly square, and connect them to the existing map highways with bridges.

They should add 4 cities to the map, one at each cardinal direction.

North of the map should be a Detroit inspired city, very industrial and cold.

East should have Manhattan style super dense city of sky scrapers.

South should have a Miami style with beaches and palm trees and islands connected by long narrow bridges.

West should have a San Fransisco style city with mixed urban and dense, with lots of hills and vertical to the streets.

But stylized to really be their own thing, not scaled down exactly from the real cities. Lots of verticality, jumps, alleyways, roof tops, parking garages, train tracks, subway tunnels, all that cool stuff.

And they could make each city its own special expansion pack update event, with themed cars and stuff. North could focus on old school american muscle and luxury cars from the 50's to 70's, East could be german inspired modern sports sedans, South could be 80's supercar inspired, and West could be 90's cusomized jdm and suvs.