One of the big things that KeeperRL is still missing is a good system of capturing and exploiting prisoners. I'm committed to fixing that in the coming patch, although I don't have a very clear vision yet. I'll lay out what I've come up with so far, and I hope to get some feedback from you.
At the moment prisoners can only be captured inside your own dungeon, and the rules of how it's done are hidden, so the system is unclear and random for the player. The main goal is to make the requirements explicit, so it's clear what you need to do to capture a prisoner.
The main approach that I'm looking at is using the immigration UI. In the example screenshot above the player can schedule capturing of prisoners, provided that the prison is big enough. Once the orders are placed, minions will try to capture prisoners at the nearest opportunity, for example when raiding a village.
The disadvantage of this system is that if the player forgets to place the order before a battle, they will lose the opportunity to capture anyone. It's also unclear how captured prisoners should travel back to your dungeon. Should they join the raiding team and return with them? What if you want to continue raiding, are you going to drag the prisoners with you to the next battle? They could also return by themselves, but it wouldn't be realistic - they could easily escape.
A much simpler idea is to implement prisoners as regular immigrants. The two requirements would be building a prison and capturing a village. The remaining civilians in the village would, at the press of a button, turn into prisoners and come to your dungeon.
Just like above, having prisoners travel alone would be unrealistic. The idea also seems too automated and lacking any challenge. The system also doesn't easily accommodate capturing non-civilians. One major advantage is that it's very easy to explain to and to use by the player.
There are some vague ideas of gamifying capturing a bit more, so that it requires more effort or strategy from the player than just killing enemies. A special "slave master" minion could be the only one capable of taking prisoners or there could be a special weapon, like a throwing net, or a trap. But I'm afraid of making the system unwieldy and requiring too much micromanagement.
Once prisoners are captured, we need to decide what to do with them. The current options are execution, which aggravates enemies, torture, which produces mana, or having them supplement your imp force. Every captured creature turns into a generic prisoner minion, and they are all identical.
There are a few ways to make things more interesting. One is letting prisoners retain their identity, making some more useful at certain tasks than others. Or even letting them join your armed forces. I think enemies should also try to break in and free prisoners.
I was also thinking about making prisoners more essential for progression by limiting the capabilities of imps. For example after some length of digging the rock could become harder and only possible to mine by prisoners. Same thing could be done to some or all minerals. Acquiring a large number of prisoners would thus be critical to have a successful dungeon.
All of the above are just ideas floating in my head or being partly implemented. I'll do a lot of playtesting before I settle on a solution, and I also expect that things will need to be refined in later patches. Hopefully I can come up with a system that will add a lot of meat to the game. Meanwhile, I'm eager to hear your thoughts!