First off, this is not the place to discuss slavery in Fortress mode. That has been discussed to death. If you want to talk about it, do it elsewhere.
Okay, as we all know humans have the tag [ETHIC:SLAVERY:ACCEPTABLE] and will take slaves during worldgen which can later be discovered in Adventure mode. However, I find that these slaves are being poorly handled since adventurers are able to just walk up to one and hire them with no repercussions or difficulty. I believe that this can be refined in a few ways:
1. Ownership.
Currently, "slave" is just a job title that members of other entities might get if their civ is conquered by the humans. They have no owners, they just stand around with everybody else. I assume that they are just owned by the entity instead of individuals. In the short term, I suggest that it becomes illegal/impossible to just walk up and hire them. In the longer term though, it should be possible for both adventurers and worldgen figures to buy or liberate (or buy and then free) enslaved people from their owners. If the entity owns the slave in question, then they should be purchased from a local representative, registered dealer, or the lord/lawgiver/etc. who would likely have a standard price based on age, physique, whatever. If it is a transaction between individuals, then other factors may come into play: attitude towards eachother, relationship between slave and owner/buyer, and other things that can raise and drop the price.
2. Appropriate Legends Info.
Different entries in Legends mode entries based on who owns them, what they are put through, who enslaved them, who liberated them (if applicable).
"In 324, Cacame was enslaved by the human Ozi Slaveowner"
"In 324, Cacame was forced to become a farmer in Wallthunders."
"In 953, Cacame was liberated by the elf Lari Arrowspotted during the raid on Wallthunders during the Aggrieved Conflicts."
And so forth.
3. Escape/Revolt/Suicide
Being a slave sucks. No person wants to be a slave. As such, escape attempts should be made by the slaves. If they are treated well, then fewer attempts are made, and if they are poorly treated more attempts will be made. Get enough angry slaves in an area, and they just might band together to create a revolt against their abusive masters. But escape and revolt aren't the only ways out. Depending on their treatment, their beliefs, and their personality traits, some (though it should be a fairly small number) may prefer to just end it all instead of running or working.
4. Treatment.
As stated above, different masters will treat their slaves differently. I define treatment as housing and food quality, as well as work hours, types of punishments used for misbehavior, what is/is not considered misbehavior, and kind of work engaged in (slaves used as "target practice" aren't well treated). This may be based on history (a slave owner's family was slaughtered by goblins, he then abuses any goblin slaves), personality (they just love that elven grace), religious beliefs (may involve imposter deities and their antics), or other factors.
5. Differing Opinions Within an Entity.
Naturally this is a pretty long-term goal, but worldgen figures should have different ideas about how slaves should be treated. This sets the stage for internal conflict, neighbor vs. neighbor, and possibly the use of the military to put a stop to the abuse/coddling of slaves in a town. A false deity may love slavery, while a local lord disagrees. This sets the stage for a lot of interesting (and very entertaining) strife to happen during worldgen and generally enliven history a bit.
6. Variable Labors.
Not all people are made for physical labor. Some races would simply not know how to do a lot of things. Kobolds, for example, don't have the ability to farm and wouldn't be much use in that job. However, they are able to carve bones, hunt, and trap animals. A Kobold slave would be better adapted to working in a sweatshop carving bones from the butchers into crafts for their masters to sell. This should be reflected during worldgen by having them placed in professions that they would have in their own civ, since it would most likely be too much of a hassle to try and train an elf to chop trees.
7. Markings/Maiming/Restraint.
How would one tell, when walking through a human town, who is the slave and who isn't? Why, slaves would be marked, of course! The marking would have to be visible without the removal of clothing, so slaves in colder climes would likely have markings on their clothing or on their face or in some other position of prominence to avoid allowing them to slink away in a crowd. But that's not all, if a slave is especially willing to run - and particularly if their job does not require a lot of moving around - the owner may opt to have them crippled or heavily restrained to prevent future escape attempts. This ties back to how the owner feels about the slave(s). A cruel taskmaster may just hobble a slave at the drop of a hat, whereas a kind (relatively speaking) master would just chain them to their workstation until the end of the workday.
8. Different Sources of Slaves.
As it stands, slaves are only gained through warfare. While there is certainly historical precedent for this, it's not the only option. Goblins will kidnap and imprison children from all over, so I think that the humans should be able to buy theirs citizens back and even purchase other people from the goblin empires for use as slaves. Other human empires with more wars under their belt may have an excess of slaves and would be willing to sell some to any interested surrounding kingdoms. A slave trade would also allow for slave caravans traveling between towns/empires once caravans actually travel across the worldmap. Another option would, naturally, be people born into slavery. Whether they were pregnant at the time of enslavement, started a relationship afterward, or forced into breeding to sell the children (horrible, but it's what happens, unfortunately), born-into-it slaves can end up supporting a kingdom's whole supply in the absence of war.
Once again, I'm not discussing slavery in Fortress mode. I would just like to see a few of these things occur during worldgen to help add character to my worlds and, later, to my adventures.
Edit: expanded #8.