I have written one other, poorly written, suggestion on the topic of slavery and what it can do to the immersion of the overall game-play. I'm going to get straight to the point.
-Slavery In Dwarf Fortress.
Heres how I think it would work
Throughout time slavery has been an inevitable part of history. During of which slaves would be acquired. War, Trade, and Prejudice. Which brings me to my first point of slave acquiring
War.
Slaves of war. This is how slaves are acquired without paying for them. to get a slave you would first need to declare war on another nation (Lets say elves) once that happens and they siege you, you can obviously attack, or wait. When you attack you select a option to "Maime" the elves without killing them. This would lead to a sudden need of doctors. With your new captured elves you will have to fix them up and heal them before they are put to work!
Trade.
Trading of slaves. This is the easier, more honest way to purchase, or sell slaves. This can lead to profit, and prices should vary depending on race, age, strength, ect. The slavers would come to your fortress every couple of months. (The more slaves you purchase per trade, the more often they come) Similar in trading, you should be able to order certain slaves by priority. Set priority by race, age, or strength.
Keeping.
Keeping slaves in line, and away from freedom, is not an easy task. To keep your slaves you would need the following items. Chains, copper bracelet, and a neck bracelet. PER SLAVE. You would also need a primitive sort of housing to keep them in. The best method would mass store them in a big room with a cot for each slave and guards at the door. You also need to feed and water them constantly, (More than dwarves from their hard labor and suffering) making it difficult to own and upkeep slaves.
Moral.
Even slaves have it. Many factors contribute to moral, which manages how efficiently they work and their likely hood to revolt. The first factor would be food and water, those two simple things can make a huge difference. The second is housing, believe it or not, slaves don't like being kept pact in like sardines. (even though its more efficient) The last is labor, if you keep them working to long or have them weighed down with too many chains, they will become bitter. [Suggestion; don't pack too many chains, and keep work hours to acceptable level]
Labor.
Slaves are dumb. They cant do skilled labor. They would be restricted as laborers for moving stone, mining, cooking, stone grinding, and farming
Revolution.
Viva Le revolution! Yes, slaves don't like being slaves. So if moral gets low enough they may want a "Change in ownership" for that too happen you need several factors. A; TERRIBLE moral. It has to be serious. I mean 24 hours of work with minimum food or water in full chains. B; Weak defense. Slaves can sense a weak military. If you guard the quarters with minimum guards (1-2) they may try to overpower them and steal their weapons, and release their friends and start a riot, in an attempt to destroy your fortress and everyone it and run it themselves. The final factor is C; The killing of their pals. If you go around killing slaves you don't like, that's a no-no. They will be inspired by their martyred brothers and attempt to avenge them.
Extra Facts;
-These Revolutionaries will have clear leaders (1-2) Once they are killed the moral of the other revolutionaries will drop and can be squashed more easily.
-Once they are freed, the prisoners should have their moral skyrocket to an all time high (Until their brothers are killed)
-If you have a skilled diplomat, you can use war slaves to possibly barter for supplies from your enemies
-Slaves can be given to impotent people within the fortress as servants, bodyguards
This suggestion has been submitted by TheMrJACKman12
Thanks for your time and I appreciate feedback
I am also aware that slavery is occasionally a delicate subject, that's why I warn we approach this idea with non stereotypical ideas and try and keep it appropriate, but still capture the ideas to enhance game-play dramatically