Population Quality
I was writing in another thread about immigration vs. fertility rate when I considered the question of whether cities could have different grades of citizens in which higher grades offer greater benefits than lower grades. For instance, there might be 4 grades of citizens: new migrants/slaves, freemen, skilled artisans, and Enlightened Folk.
New immigrants from the wastelands and slaves would only offer a tax income and give a negative modifier to their own base production rate. Freemen yield tax income and give a neutral modifier to production. Artisans might yield a tax income and a positive modifier to production rate. Enlightened Folk might yield tax, generate wealth, give a positive modifier to production, and give a positive modifier to research. Depending on where you get your population, how you treated them, and what services you provided, you could gain citizen types at different rates.
The feature wouldn't have to be overly complex and wouldn't have to bother the interface with anything more than a 4 numbers and a small scroll over description, but would offer some interesting flavor to the game.
I know some of the Europa Universalis games tried something like this, but failed to make it work very well (Enlightened citizens were less helpful than slaves). Any thoughts?