How about captives?

Sometimes war is not all about destroy enemy soldiers, it's about triumph over another empire. Captives is a major part of it too.

Just add a new choice at the battle window: Capture hostages/Fight for your duty/None shall survive.

Different option will change the outcome. If you tend to keep captives, you troop will try not to deal crushing blow on enemy units, and thus decreas their damage, but more percentage of enemy will be capture. If you select no captives, your soldier will try to totally eliminate enemy, that will make the combat more easy, but no enemy will be capture alive. If you just order them to fulfill their duty, then they will just do things on avarage level.

With this feature, you can add some new politcies, such as enslave captives, or kill them in public, or treat them well, interrogate by all means(you can choose), or even assimilate them for an anti-propaganda.

And of course new diplomancy actions: exchange capitives, release captives, or ask for peace via hostages.

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2017-1-28 update:

thanks for the inspiration from dragons111888:

I think this system should be used on enemy troops, because you cannot really kill a hero in this game. Even the heroes failed to infiltrate your system will be dispatched right after it. But I do figured out a way to describe how does it works:

I think the people you capture should be a score in diplomacy screen. For every, like, every 100 capitives, you can assimilate them as a pop unit, if you have proper laws, like something from the pacifist party. You can enslave them, and that will provide industral bonus for a short time over your empire. You can use them on expirments to provide science bonus as well. You can release them on certain conditions, you can name a price, or make an diplomacy proposal and use release capitives as a part of your offer. You can interrogate them to reveal some information from enemy empire. You can kill them in public to please the martialist party, and intimidate your enemy. (Perhaps they could even be turned to food, but that was...too barbaric.)

What ever you do on capitives, even the way you fight(you keep capitives or you kill everyone) will have a diplomatic impact on other empires. People might see you as a merciful leader and be more likely to cooprate with you; or people might see you as a tyrant and they will obey you under your terror. But of course, that only takes part after you won the battle. If you lose on it, they will see you as a weak and naive enemy/a barbaric threat that must be eliminated quickly.