@Grautvornix @Thalatta
Since this is the largest dedicated Slavery thread, lemme just list the options presented here (at least that are viable, if there are ones I missed lemme know)
1: Train Slave Units at Market/Dock/Storehouse - This would mean a custom slave unit, who would probably just cost metal, be easy to train, but might have some other problem (such as a limited amount that can be trained/limited lifespan)
2: Convert % of Slain Units to Slaves - This is viable, it adds little micro, though there might be problems with phenotypes (turning a male soldier into a female slave), OP usecases (snowballing is increased, since winning battles helps your eco), and actual usefulness (slain units would transform on the spot, meaning they would be slain by formerly allied soldiers quickly).
3: Unique "Slavemaster" Unit - This idea is that a unique unit would be trained at the Market/Dock who can capture other units, turning them into slaves. The micro required for this might balance it's relative bonuses of free workers.
4: Capturing Units - Similar to 3, this feature would have any soldier have the option to "enslave" an enemy unit below 20% health. This is probably OP (could result in capture rushes, where captured slaves are used to weaken uncaptured units) and is A LOT of micro.