Beyond converting other GTA cities, modders have attempted real-world cities. NYC, LA, London, Tokyo - mostly partial, occasionally substantial.
What's been attempted
New York-coded modlets. Los Angeles overlays. London street remixes. Tokyo neighbourhood-scale rebuilds. None reach the polish of the official cities they reference.
Real-city mods have copyright complications - architectural copyright, brand copyright, sometimes geographical naming - that fictional-city mods don't.