I understand you but I think the less is more here. Represent African Slave trade microscopically with actual Slave unit and Classical Period Slavery macroscopically with the vanila Slavery civic allowing to build buildings with whip and adding to it special bonuses for slave related improvements such as mines, quarries and plantations. See, the easiest suggestion would be to adopt very thoroughly implemented slave feature in RFC-CW. Slaves there act as mini-GE, mini-GM, slow worker, can be settled to become house slaves, etc. But RFC-CW focuses on specific time period and specific map segment. For a global mod and 5000 years timeframe civic works better than slave unit. Maybe its less exiting but it does represent macroscopic nature of global mod fairly adequately! No need to run into so many potential complications for very dubious gains -- Slavery Civic is much more elegant but still very strong and different mechanic: the only time one can use infamous whip and get those annoying slave revolts. Vanila Slavery mechanics is near perfect, please leave it as is.