You know what'd be neat? A class of sub citizens who you can care about even less about than a immigrant potash maker. A caste that works tirelessly and receives little besides sustenance. Sure, their work may be shoddy and they simmer with the rage of a opressed sub-proletariat, but their deaths mean little and their sacrifice can create wonders.
What I'm talking about are literal Slaves to Armok.
You know those Goblin Snatchers and Kobold Thieves that attempt to sap the lifeblood of your fortress? Why not capture them, give them a little 'conditioning', and set them to work for their sins.
And if the Elves start causing trouble? Why merely execute them when making them build the same megaprojects they loathe so much would be so much more karmically satisfying?
Maybe different slaves could have special abilities. Elves could harvest lumber in the way of the fair folk, Kobolds could subsist on bones and are especially compliant, and Goblins could... well, I'm sure they could do something. Maybe make good nannies?
Remember, though, that we could have a more Classical slavery. Slaves are captured and without rights, but so to could Dwarfs and other noteworthies be subjugated for crimes. Furthermore, slaves could have the capability to become full citizens and serve in that capacity.
Also, imagine the FUN of attempting to put down a slave riot when they outnumber your dwarves three to one and you only have five military men.
So what are your hopes for the possibility of slavery?