Building an Empire

This suggestion may complicate things more than they make things interesting, but looking at the image provided, the borders between the lands of the city-states could be changeable between the years. In that the rivers flowing throughout the country change from slightly to greatly each year, as the water flows down from between the mountains onto the plains. Perhaps the earthquakes are prone to surface-changing earthquakes, or a strong presence of beavers, or the mountain areas are easily eroded by water passage so fluctuate easily.

Putting in my two cents, it seems like a pain in the ass to balance.