It was idea based only on my very limited knowledge of history, but little reading of Wiki and technology/process can be called "Devshirmeh" and units can have "Kapikulu" promotion, if we want to stay in Turk terms.
Yes, in history it was Roman and Turks, but i was thinking about it more like something to do with slaves.
Many civilizations/nations where slave-heavy, but use for slaves is dictated by needs of civilization, so if civ needed labor/population or army or entertainment it used slaves in such roles and game should reflex it.
And adding Slaves to city population? If not like specialists (like someone wrote), than like normal citizen but with Slave/Barbarian culture (for checkings for unhappiness, revolts, diplomacy drawbacks and so on).