I like the idea of returning Slavery to Civ, if only because it's one specific mechanic that would force you to balance negative and positive factors.
In general, slave labor increased productivity in ancient/medieval eras, less in later eras, but always resulted in concerns (sometimes Major concerns) about security - slave revolts, slaves running away, slaves joining the enemy in a war, etc.
So, I suggest that you get slaves from razing/seizing enemy cities or seizing Workers/Settlers. Certain Civs might even get more slaves from a razed/taken city based on the Policies they are following - both the Tradition and Honor trees have policies that could apply. Too many policies from the Liberty tree might even make slave holding almost prohibitive in your Civ as incompatible with citizen stability/happiness.
Slaves can be used two ways.
First, as Workers. Possibly working less effectively, but certainly also requiring less upkeep.
Second, as 'specialists' in a city, providing a boost in Productivity/Gold and maybe even in Culture (slave artists, craftsmen, dancers, singers, etc) BUT never in Science.
Slavery becomes less and less 'profitable' both as a result of Cultural Policy changes (Liberty tree, mentioned above, certainly the Freedom tree later) but also as a result of technology changes - slave labor does not work well in entrepreneurial/capitalist societies, because the slave owner has to invest in relatively expensive training to make the slave worthwhile, and cannot simply 'turn loose' the slave when the work requirements change. Not economic. Therefore, slavery will get less and less useful as you progress into the Renaissance and Industrial Eras.
On the other hand, virtually every Totalitarian Regime of the 20th century used slave labor, so taking that path would allow slavery much later, but also have a major negative effect on relations with non-slave/totalitarian city states and civilizations.
All in all, it fits right in with the mechanics proposed for BNW, but I'm not sure that the proposed concept of a Brave New World of international trade and diplomacy would include Slavery in the eyes of most people.