Yeah, I could see a whole series of techs based on slavery... you could have a Slaver's Guild building that increased hammers maybe, while adding to unhappiness?
You could also have a slavery Civic increase the happiness of thunderdomes.
In general, I think the benefits of slavery should be greater than they are in vanilla Civ.
But I also think there should be more rewards for avoiding slavery. Maybe an abolitionist Civic that increases unhappiness in Civs with the slavery Civic, while hurting your diplomatic standing with them.
The abolitionist Civic could let you build an underground railroad building, giving you happiness and allowing for a chance at escaped slaves coming to join you from other Civs (maybe like refugees?)
But again, I think this would hurt diplomatic standing with the Civ the refugee came from, meaning you'd have to be careful, going abolitionist should eventually lead to war with slavery-minded Civs.
Just some ideas, I am not a modder and have no idea how hard these would be to implement
Chuck