I've thought about the addition of slavery before. I don't think just having a slave unit would really be enough, but I've never been able to come up with a satisfactory model for it. The best I could come up with is that for the civs that chose to make use of it (not every civ has made use of slavery historically, but most have), it would make production cheaper but it would cause a certain degree of unhappiness. Then there'd be a way to abolish it later. And after long enough, any civ who hadn't abolished it could maybe have a horrible reputation? I'm not really sure. This is one of those things I'd be glad to see added if it was done right, but it wouldn't bother me if they decided not to mess with it, either.