Aside from other GTA cities, modders have attempted to recreate real-life cities. NYC, LA, London, Tokyo - generally partial, sometimes mostly done.
What's been attempted
New York-coded modlets. LA-themed overlays. London street remixes. Tokyo neighbourhood-scale rebuilds. None have the refinement of their official counterparts.
Mods that use real cities have copyright issues (architectural copyright, brand copyright, occasionally geographical naming) that mods using made-up cities do not.