I was wrong about the effects of an Age of Prosperity. Without a quintessence cost, any number of cities could be created. If we make city creation a power component instead, then the emergency override Kevan created for Powers should keep things reasonable.

Create a Global Might Power Component:

Land: Create a City. The Incarnation of the Deity using this Power is its Founder, and the Deity may choose its Name, Dominant Pantheon and Description. (+15 Global Might)

Change the Cities rule to read:-

Each City of the world has a Name, Founder, Dominant Pantheon and optional Description, tracked in the Cities Wiki.