and just many empires collapse when they expand beyond their means of capacity.
an empire eventually winds up having more population than resources for that populace, causing strife and internal disparagement of it's citizens. Rome became so big that only conquest could "feed" it's "hunger" and the bigger it became the bigger the hunger became. Eventually there was nobody left to conquer, no more external resources. Rome began to collapse, as the merchants squandered the resources and nobles were sent into poverty.
As it collapsed, and as with any nation, fragments of the populace drift away into rebellion or to relocate to a place with some "pull factors". When the merchants relocate, they take the wealth with them, which leaves the nation further impoverished and eventually it falls apart.
So, a game that has something like Stability (look up RFC) to encourage a nation to not fall into the same trap would be a good thing. Some could argue that in todays age (and in the last 100 years, especially the United Fruit Company case or the case with Patrice Lamumba) the first world has found the third world to be nearly defenseless against it's technology and still relies on a form of third world slavery for its own wealth.
In theory, if a forcefield appeared to block first world access to the third world, the first would be pretty screwed pretty quickly.
To keep on track, a form of stability and emigration would be nice to see. When a citizen migrates, it would take a percentile of the treasury (relocation of wealth). This way when people leave miserable nations to happy ones, they take some of their wealth and stimulate the economy in their new homeland.
In addition to something bad for unhappiness (revolution) there would be something GOOD for happiness (golden ages) and Immigration would enhance this by causing both population (and the food that made the citizen!) and some wealth - rewarding the happier nations with noble families relocating.