I think slavery is an institution that has been dealt with almost every great civilization. I think there should be some way of dealing with this issue in the game more directly than stealing your enemy's workers or being able to trade workers that are in a civ's capital.

One thing that I suggest is that if the institution of slavery is accepted in a civ's government, then defeated units of an enemy would turn into a worker or workers, much like what happens to a captured settler.

Slavery could be one of those things you can turn on or off like having a Mobilized Economy. Having it turned on could mean that citizens in your cities that are ethnically from an enemy civ would turn into workers, and workers under your control that are ethnically from an enemy civ could not join a city and becme a citizen. So having slavery turned off would allow them to join a city and be a citizen.

Don't forget that slavery not only already exists as foreign workers and trading workers, but also as pop-rushing. At least that's how I interpret pop-rushing; forcing the citizens to work till they're dead (or escape), like in labor camps. Of course, I guess not all slavery is labor-camp slavery.

If being a slave nation means getting a worker every time you kill an enemy unit, I think that's too strong. Maybe it could be like this:

Slaves work at 1/4 (maybe 1/3, or even 1/2) of standard worker capacity. You can only have the choice of becoming a slave nation, or abandoning the slave nation, during a revolution. Allowing slaves makes your populace somewhat unhappy, depending on your government. It matters the least in despotism, and the most in democracy. In the middle (in order from least to most) is monarchy, communism, and republic. You cannot use slaves under anarchy, since they are unwilling to take orders from you. Other civs that don't allow slaves have a lower opinion of you if you do allow slaves. Also, slaves cannot join your cities, as you said.

Just an idea though; I'm not 100% sure I'd want this in the game. :)

Personally, I think there should be a wonder/minor wonder that, while keeping the foriegn nationality, enables captured workers to act as a common worker- they have gained thier citizen ship, so to speak

I don't think that slavery is hard to incorporate.

However, since land, combat units can possibly turn into workers or a new slave unit when captured, then perhaps creating combat units would also cost a life of a citizen in the city.

This would be unlike creating actual workers or settlers that when built they cost one or two citizens and when captured they turn into workers for the other civ.

I already tried to incorporate this in my mod by making certain units cost the life of a citizen, but when captured they don't turn into workers.