This is, in part, related to capturing champions/heirs. I would take that much further. I'd like for players to capture soldiers, villagers, livestock, women and children, you name it, and do with them what you will. Ideally, you would be able to round up whole city populations and march them to camps back in your territory.
This is also inspired by the sacrificial altar building, which as of now just provides a bonus and costs prestige. I'd like to be able to sacrifice captives on that altar. Make it more like a powerful enchantment that requires human souls to fuel. Maybe the bonuses it imparts keep getting more powerful the more people you kill on it. It could even prefer innocent people, like farmers or women and children.
Morality in games is sort of a trend now, but you don't often see opportunities to roleplay a truly evil character. There are opportunities to impart some sense of moral consequence through scripted pop-up messages, AI faction reactions, anything you can script. It would be nice to have even more of that, but the important point of this idea is to just let the player be evil.
Think of all the ways captives could be used as resources:
- Require X captives for the construction of a gladiator arena
- Require X captives for the construction of a slave galley
- For the construction of slave caravans
- For the construction of a Vlad the Impaler-style wall of crucified and beheaded corpses around your city
- For transforming into mindless zombies or monsters
- For the construction of a harem
- For use as live training dummies to level your soldiers
- For developing intelligence (X enemy soldiers to use an Interrogation Room building)
- Many more posibilities
This could also be a way to differentiate factions or races. Think how much easier it would be to justify hating the evil races if you knew that only they were capable of doing such evil things (though even the "good" races would hold captives for ransom and other medium-evil things).
Once you have a system of taking captives and counting them as a resource, you open up a ton of possibilities. From there, though, you could expand it to rescuing captives, using an enemy's captives against them (e.g., slave uprisings), tracking the captivity experience of individual champions... All this to give the good guys some gameplay value out of it, too. Giving some players the opportunity to be evil gives others the opportunity to be good. And it enables all sorts of interesting things to happen.
Imagine some of the stories that could develop out of this. Wouldn't it be sad to learn that your heir was captured by the enemy and sold into slavery, but then awesome if he himself led the slave uprising that brought down the enemy's capital city?