Workers and Slaves

Per the previous suggestions, add a "slave" resource on the map in certain areas such as sub-Saharan Africa. I'm sure there would be an interesting tech or civic required to reveal the resource deposits on the map.

Then, the "slave" unit can be built. It's similar to a worker but it requires access to the slave resource. The unit can then do whatever you want it to do differently from a normal worker.

I think that this resource-based system is superior because resources are fundamentally based in geography. They're also easily tradeable, so that solves two of the biggest problems with getting access to slaves. Instead of worrying about who killed what unit where, all that matters is whether or not you control the slave resource or trade for it.

Building slaves in a city might seem weird, but it's the same as building a war elephant when you have access to ivory.
 
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