An idea inspired by the "Hold yo Block: Total War" mod and of course "The Godfather"
Only problems I can think of are battles and automatic weapons. Basically, cities are now rackets, and population is replaced by customers.

Factions:
Corleone family
Tattaglia family
Barzini family
Stracci family
Cuneo family

The map would be NYC during the late 30's and forties.