We know that in worldgen people get abducted.In Fortress Mode babies get abducted by snatchers.And in Adventure mode you will receive quests to rescue the king's son or whatever.
Based on this info we can say that the abduction system is pretty complex,but I think there is something missing.
The basic idea is that when the player gets ambushed and fails to beat the goblins he will get knocked unconscious and the goblins will bring you to their fortress instead of brutally murdering you and your companions.After that,you will awake inside a "cell block" deep inside the fortress's structure naked,unarmed and severely wounded.You will be forced to follow orders from the wardens like "Follow me,human trash!","Carry those bodies,you dirty slave" and even say some really offensive shit to specific races like:"You know what you really are,Urist?A DRUNK,FAT,HAIRY AND TINY PILE OF SHIT!","You know why you will never be manly,Cacame?Because YOU ARE A BLOODY ELF AND ELVES ARE A BUNCH OF GAY IDIOTS THAT CARE ABOUT TREES!!".(I was going to write something about kobolds but it was too heavy to post here)
The player will have to choice what they want to do about their imprisonment:
-Make a escape plan with others prisoners.
-Accept their destiny and become soldiers in name of the goblins.
-Make a prison riot to takeover the site.
-Create another character to save the original one.
In addition a abduction system like that would make the players care more about rescue quests and add new quests that aren't just about killing things,like saving your friends or family members.
I think it would be a nice touch to see abductions from the victim's perspective and see what actually happens in Dark Fortresses.