History does provide a lot of ideas, but I have an impression that when it comes to the topic of slavery, there's a trend towards taking ideas from a very narrow time and place in history, and ignoring opportunities to use anything else. There are many historical segments - Greek, Aztec, Chinese, Soviet etc. - that handled it differently than in USA, but those conceptions tend to be overlooked, and instead portrayal near-always leans towards the USAish, even if 'historical' circumstances of the setting wouldn't naturally lead to it being that way.