Small suggestion about new slavery system:
a). "Slavery" civic gives every civilization ~20% chance to enslave defeated enemy unit. These units can be traded to another civilizations etc.
b). If civilization runs "slavery" civic, then every slave unit can:
- Upgrade to worker unit or warrior, or settler (need some costs in gold)
- Hurry production in city (+90 shields),
- Join to city population, as a freedman (+1 population),
- Join to city as specialists (+2 shields, +1 unhappiness)
- Build a Slave Plantation, or Slave Farm, Slave Mine, Slave Quarry, etc.
- Every using of slave unit will consume this unit.
c). Slave planations, farms, mines, quarry'es:
- Gives effects only with running a "slavery" civic. If your civilization does not running "slavery" civic, these improvements will not effect in any way.
- These effects are additional yield shields (+1 of production, commerce, food?),
- They also giving a 0.25 unhappiness per one slave improvement in city radius.
- Can be transformed to regular farms, mines, plantations etc., in one turn, but only with using a worker.
- If you want to upgrade your civic to, for example "Agrarianism", you should be prepare for it, to avoid economic crisis. Your Slave Improvements will not effect in any way without running a "slavery" civic.