hey, i really like this idea of expanding the usefulness of a slave economy
i was wondering if it were possible to create slave specialist slots in cities with slaves acting as mini 'great people' and adding themselves as a free slave specialist using the same mechanic.
they could add hammers (and gold with slave market building?) and detract from the food in the city in a similar way in which they are implemented in the Rise of Mankind mod.
if the civics are switched from slavery all the specialists are lost (you've emancipated them) and you'd have to start again from scratch if you returned to slavery. if you've spent a lot of time capturing lots of slaves from your enemies it provides a good long term benefit if you have the food for upkeep, and it also means an incentive to stick with slavery if you plan to keep this economy going.
makes for a bit more than just the +10 hammer you get from rushbuilding with slaves, but only really pays off if you plan to stick with slavery for a while.