African slaves should be available from an African screen for Trade Goods, and Muskets. Equivalent to a normal Colonist for outdoor work and 2 units for indoor work (freed Slaves) equivalent to an indentured servant, reflecting prejudice and lack of education. Send ships to buy them from Europe to Africa then America: the slave triangle.
A special type of Colony called a Plantation should be available to build. Instead of a Town Hall it would have a Planters House which could accommodate an Expert Planter only, the Plantation could only produce his commodity. He would produce Liberty Bells in the same way as Statesmen thus increasing production over time. An Expert would double the production of his speciality crop. All buildings would accommodate only 1 Colonist. Planters could work as normal in Colonies.
Colonies could still produce cash crops but not use planters to harvest them. This reflects the underdevelopment of the Southern United States and West Indies and makes defence more difficult due to reduced lumber and SOL production thus losing you cash crops during the war of independence. George Washington and Thomas Jefferson had Slave Estates so SOL production is accurate, despite the lack of liberty for the said slaves. There could be a chance of a Slave revolt especially if there were no military units in the Plantation. William Wilberforce the British anti-slavery campaigner could be included in Congress, to outlaw Slavery very late in the game if you still haven't declared independence. Liverpool and Bristol grew rich on the slave trade, as did many other ports and countries.
You should be able to free the Slave unit to become an Indentured Servant unit (due to lack of education) which can never be employed on a Plantation again. Only Slaves can work at outdoor work on Plantations.
What do you think about including slaves in a Civ style game? 3.64%: It would be immoral to include slaves. 87.27%: Slaves were are part of history and should be included, it would be bad to (indirectly) deny slaves existed. 9.09%: I wouldn't find it immoral to in- or exclude slaves. Voters: 55.
[ So production doesn't increase for when Thomas Jefferson, Thomas Paine and Simon Bolivar join the Continental Congress. This would make Plantations less efficient over time as compared to colonies with high SOL membership. ]
In fact, not only the experts should double the production, but all the slaves should do that, first because they were in very big number, second because if they produce the same than a colony no one would like to build a Plantation.
Plantations should be created within 3 squares of distance from a colony (so a plantation should always be under the leadership of a colony), the number of slaves must be proportional to that of citizens of the colony (for avoiding to have a white and a lot of plantations).
[ This would make them close to a place with Warehouses and a Custom House for trading the vast quanties of Tobacco, Cotton and Sugar they produce. I believe Dye and Rice were also grown in quantity. ]
[ Free Colonists and Indentured Servant work the same for outdoor work. It's only at indoor work that they differ, 3 production for the Free Colonist and 2 production for the Indentured Servant. The Indian Convert only has +1 outdoor production and this is due to their knowledge of Tobacco Planting, Cotton Planting, Sugar Planting and Local Foods. ]
[ Yes I agree they should be cheap to buy, -1 food, +1 (or 2 to be better than Indian converts) produce. Where do Master Planters fit in though? And what about leaning planting skills from the Indians? Mercenaries is an OK idea, but I think a better way would be to add an Africa screen like the Europe screen where ships from Europe loaded with Trade Goods arrive to trade for Slaves and then leave for the New World i.e. the 'Slave Triangle'. ]
[ Sounds like a design for a follow up game. ]