Maybe instead of costing less unkeep, slaves should be more productive than workers (as while low quality, they are many more people) while keeping their chance of dying.
I have thought of a couple of approaches to slavery, don't know how implementable they are, though:
Slaves as GP: slavery enables the build of the slave market "wonder",that spawns slaves every now and then (would be good if it depended on you being at war with other nations, fighting barbs, etc). That slave unit is sent to a city where whether it can be tooled to work on the fields like a worker, join the city (i.e. in the form of a little slave market/house with income modifiers) or rush a building.
Slaves as resources: captured cities, or cities with several culture types can build a slave market that provides slaves, which could be used to build buildings like slave markets and colloseums, traded with other nations, etc.
Slaves as specialists: slaves are fixed specialists that only cost 1 food and provide hammers. Many of these rise the chance of a slave revolt. Several turns of slave revolts and they'll destroy something in the city or get weapons and destroy what they find in their way to freedom.
Slaves as a mixture of all the previous implementations: slave resources that enable the slave market wonder that spawns slaves GPs that join cities as slave specialists.
Some other ramdom ideas:
Revolting slaves probably aren't military units and have no way to supply themselves, so they should probably be unable to attack (effectively) a city, nor (most likely) be able to survive after destroying an improvement. Then add slave leaders: Slaves with military training that are stronger and can supply themselves. They can gather slaves (somehow) and turn them into warriors for your nasty Spartacus style uprising.
Would be nice if (legal) ways to free slaves (like Rome's freedmen) could make the slaves a bit happier and keep them from revolting too much. Slaves should revolt because they are strong enough and are unhappy, not just because they exist.
With a couple of changes, most of this could be used in forced labour and detention/forced labor camps