I think we should have a building called "slave house" and "slave market".Slave house give you the slave pop but you just can build 5 houses and 10 slave per house when slave market allow you to trade slave
My idea is 2 or 3 elite units can arrest one enemy's female and make she become their slave.People can choose to kill or make them become slave,it will take few seconds to arrest and just possible if female's health below 20% that what call "Slavery system"