Unescorted Slaves are Fine, IMO. You don't need a military detachment or legion to guard slaves -- the Slave unit includes guards in it. You should even make them 1 MP instead of 2 MP that free Workers enjoy -- to show that transporting Slaves was not as easy as free Workers (and to emphasize the difference between those two). The code will get too slow if you look for unguarded Slaves -- no need to micromanage them to such a degree. Just tie the revolts to stability and Slaves-to-population ratio...
Perhaps when you sell Slaves to the other civ they must get that actual Slave unit. So you need to think twice and also welcome other civ's slaves visiting your cities to be sold there. AI need to learn to benefit from Slavery dynamics...
Slavery civic can allow to "train" slaves -- certainly there were people who were born as slaves. Or perhaps one of the Civ's UP could be the ability to train Slave unit. We also need a nice unit graphics for Slaves to distinguish them from Workers. Perhaps Workers could get clothes and Slaves stay semi-naked, as current Workers are...
Perhaps Whipping can be replaced with Slavery dynamics. One can "whip" a building but not a training of the units. Wonders should require double slaves -- 25 hummers from each Slave. This way rushing buildings can be easier than Wonders.
In addition many UPs can be connected with Slave mechanics. One civ can use Slaves for happiness (gladiators or female slaves
), another civ's UP could be using slaves to rush units (ghulams), the third one (Qin) can get a 50 hummers per Wonders (instead of regular 25 for Wonders and 50 for buildings), etc.
Selling slaves needs to be connected with another civ's available funds, perhaps? (if they are going to take possession of the Slave unit) Otherwise if they act like mini-Great Merchants and Caravans we can think of them being sold to private hands
Perhaps there must be a counter for human player, checking the Slaves to Population ratio. If one gets too high -- instability and unhappiness will ensure.
When properly done, balanced and tested Slavery dynamic will become a landmark feature of this mod -- very fitting for Classical World
!