I've kinda got this weird idea that like maybe Slaves could have interactions between each other. Like become friends/enemies/lovers. Like two slaves could come up and ask for permission to live in the same room or a different slave tells you that a slave is execution and you are unsure of the father. Another idea is that you walk in on the slaves having sex and you could either join in, pregnant the two or force the seperation and make one watch the selling/separate of another.
Because right now slaves can only interact between each other like if you're having sex with them. I would think it would be more interesting if there were more to NPC's than meets the eye.
Just a suggestion :)