Agreed that it would be an interesting dimension to the game world, particularly if there are cultural differences regarding slaves, and factions or deities (or at least some smaller groups in-game) that have a vested interest in slavery, either pro, con or other. Also, in terms of player choice, it would also be interesting to not only be able to kill slavers and free all the slaves (as per your typical RPG hero), but also options to become involved in slaving and profit from it yourself (at the risk of losing reputation with factions, angering certain companions, etc), or keep the system of slavery intact, but work to improve conditions, etc. Or free several slaves and offer them employment, losing some of your own money to gain benefits of their work.