I like the concept of Prisoners of Wars and Slaves. Diplomatic effects it can have both ways.
And trading for captured dwarves...
Races with ethics which make slavery Acceptable... can have a slaving business. They attack your fort, by ambush or whatever, with low-lethal weapons, like whips. (Pain to knock dwarves into unconsciousness or stunning)... unconscious/stunned dwarves(even military dwarves) can then be captured with a net wielding slaver. (Like kidnappers...)
Grab 1 or 2 dwarves... then run like heck for the border! With the non-carrying slavers intercepting any close by rescue attempt...
Would be great to be able to pay some sort of ransom to get kidnapped kids and captured dwarves back... prisoner exchange too.
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Since Dwarven civilization... is by default... against Slavery... it means you, your fortress can do it. But, noone is gonna like it. Your own civilization won't like it. Give them the power to boycott trade with you. Or something... lower immigration too. The less happier dwarves can also emigrate over this. Once emigration is implemented...
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Everyone should be able to play their fortress as they see fit. (Who are you to dictate what I let my slaves do and don't do? >.>)
If I wanted to, I can order that slaves only be able to do menial hauling work. And someone else could decide that menial work includes butchery or fishing or farming. And some other could decide that slaves can do the same sort of work as Dwarves do... and that they can eventually gain citizenship/freedom...
Or just abuse/work em to death... (who in their right mind is going to give citizenship/freedom to elves?)
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Should also be able to buy slaves from slave acceptable civilizations... and be able to enslave your own/other dwarves... (big big immigration dropper and quick way to be hated/attacked by your home civilization, after failed diplomacy)
Slavery, a way to stop dwarves from emigrating... >.> A nice way to piss off their friends and relatives too...
Declaration of Independence from home civilization... over slavery rights.
And multi-race fort... I want.