Slaves worked on plantations, built cities and colonies,and even contribued to middle-class society (it was until American discrimination that all blacks were viewed as inferior.) Most colonies in the new world used slave labor to start up.
But here's the thing: Most countries discontinued slave labor, including homeland Britian and Mexico/Spain. Only the American colonists still used them excessively past 1800.
Slaves are not a part in Age of Empires III, though they WERE a MASSIVE role from 1600 to 1860 or so, depending on the region. I think that perhaps there could be an alternative to villagers that cost less food and no coin, but are weaker and less efficient at work (or maybe more so,) but as soon as you hit the Industrial age you are no longer allowed to train them.
Yes, I realize that there won't be any large game modifications since it comes out in a month, but this is something historical that ES should not have overlooked entirely.