I'm not sure it's a controversy issue so much as a profit expectations issue that would keep Firaxis from adding a slavery mechanic of some kind to BNW. They would certainly keep it low-key; we're not going to see a bullet point on the shiny BNW box that says "Brave New World: Now with 30% more slavery!" The vast majority of people probably aren't going to be excited enough by a slavery mechanic to buy the game simply for that, and it doesn't really make sense as an attempted selling point compared to all the other major stuff planned for the expansion. Therefore, resources are better spent elsewhere. Could a non-intrusive mechanic be introduced to model the effects of slavery on human history, as in Civ III and Civ IV? Yes, but it probably wouldn't be worth the time and effort to add in, and it would likely feel tacked-on. If anything, the concept might see representation as a social policy.