The problem I see with Slaves as a resource is not only that it's already in the game as a civic, but slave trade has been important for civilizations far before the age of colonialism.
So if it is to be implemented, slaves should be used from the start of the game, which might bring problems with balance and might be tedious to implement.
So if it is to be implemented, slaves should be used from the start of the game, which might bring problems with balance and might be tedious to implement.