The other I had a long car ride and ended up drawing some basic rules for running a city. The idea was really simple. Each city has 6 stats loosely based on ability scores. (might. subterfuge, welfare, knowledge-base, economy, culture.) Then it's just a simple ability check based on the ability that fits. You would customize your city by adding different building and improvements that increase the base attributes well adding on new abilities. Something like adding a blacksmith increases the economy by +2 and allows the city to a proficiency bonus when the city tries make/buy/gather items that can be made by a blacksmith. For example, if you needed to equip an army with swords. (A city levels proficiency like a player dose. Each building or improvement is basically a level.) Adding in a wizards tower would increase the knowledge base by +2 and allow the city to cast a number of city spells per month.
Kind of a ruff idea so far.