I would like to see the population divided into some sort of a class system.

For example:
Rich/nobles, citizens, freemen/foreigners and slaves.
Plundering a city would kill a part of the population, enslave a part and convert a large percentage of the rich to citizens, more rich population in a city nets more plunder.

Citizens of the rich and regular classes would serve in the army and be lost if killed or captured in battle.

The rich population would net large sums in taxes, citizens and freemen/foreigners less and slaves none, but slaves would serve as a multiplier for the taxes the rich population would pay.

Improvements in farming and trade would promote more of the citizen population to rich but if trade and farms are damaged or the city sacked they would be demoted back (some killed/enslaved in a sack).

Freemen/foreigners would be drawn to wealthier cities and slowly over the years promote to citizens. Slaves would also be drawn to wealthy cities (bought by the rich) or taken as captured enemies, and very slowly convert to freemen.