Okay, how about this instead: while running Slavery, regular slaves can be acquired using the following ways:
- capturing a city: number of slaves proportional to gold plundered (good indicator of city value and past conquests)
- capturing a worker: turns into a slave instead
- defeating a barbarian unit: ~25% chance, or maybe depending on XP value
Regular slaves work like workers, count for the worker limit, and can contribute production to a building. They cannot be traded.
Furthermore, while running Slavery or Colonialism, native slaves can be acquired by defeating native units (~50% chance). Native slaves can be traded, only civs running Colonialism can buy them. They work like workers and count for the worker limit. In the new world, they additionally can build slave plantations (+1 production, +2 commerce, +1 unhappiness) or join cities (+2 production +1 unhappiness).
Aztecs still have their special type of Slave from the UP, which they can additionally acquire from proper civs, plus probably a higher base chance.
The "Enslave" button is removed.
This way, no special rules about sub saharan Africa are required anymore. Native units are present mostly in sub saharan Africa and Mesoamerica, which are both appropriate for that kind of slavery. Classical mediterranean, Muslim or Steppe civs can still make use of their slave units without undesired interaction with the colonial trade system. The ability to use slaves in colonies is now also not hard coded to any specific civs but instead relies only on civics and location.