As i wrote in steam community, i thought that i should write my opinion down here too
A jail would be perfect for the flow of the game. when goblins take enough damage from your soldiers, (or get scared from your soldier’s sheer numbers) you can put them into jail(new building), and you can use them as hostages so goblins won’t attack you (but they will try to save their comrades) or you can use them as workers(not as good as your workers) and for them to work, you’ll need a warden to watch them(new job). A warden can watch over 2 goblin hostages but a jail can contain 4-6 goblins. Goblins you captured will eventually die so you can’t use them forever. you’ll need to capture some of them again. I thought this idea would be helpful in the early game 'cause you have less hearthlings and you have a lot of job to attend to them. You can add more flaws to this idea if you want 
Ideas: Jail = New building
Warden = New job
in the early game, your hearthlings will be too busy being farmers, carpenters, and regular workers to be jail wardens. considering the hostages will be ineffectual as workers, would constantly bring attacks against your kingdom in order to free the slaves, and uses up a precious hearthling to watch over poor workers who eventually die anyway, I’d rather just kill them and be done with it. at least then I get loot.
I’m not too keen on bringing in a system of slavery into the game, personally. while this game is in the middle ages, it’s still a fantasy with its own rules and values, and considering how hearthlings are pretty much communistic as it is, slavery just wouldn’t make sense being added in, and would leave a sour taste to some.
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Thats a spot. Thank you for your comment, you’re right about the worker thing, but think about the attacks, it’s just for saving their brethren, and their only attacking place is jail so they would obviously attack only to jail so you can take them out easy with your soldiers.
it doesn’t have to be 2 goblins that a warden can take care of, i just thought about a number thats all, that number can change anytime. Yes it was sour for me too at first, but making them hostage and make them attack only for saving their brethren(they won’t steal anything from you)is a plus.
Edit: And I just thought about it, maybe you can give the hostage to them and demand something in return. Making deals with goblins.