Surrendering and taking prisoners should definitely implemented. I could see this even for enemy factions, e.g. bandits taking high-ranking opponents for ransom, a necromancer needing live strong specimen for experiments or maybe even the odd orc wanting to gather a little intelligence. Gives a lot of room for emerging gameplay and quests.
Also, it’s way more realistic. Surrendering and POW is rarely featured in strategy games. Most of the time, you can be happy if they even implement a morale system and a possibility for retreating without automatically losing the game. In videogames, it’s basically always fighting to the death, which is neither fun nor realistic nor very rich in variety. Actual combat, in medieval times or modern, has never been like that. You can of course abstract somewhat, think Unity of Command, but in Battle Brothers I would really like to see alternatives to mortal combat done on a small scale. It just lends itself very well to that kind of features. I would love to have bandits surrender or take a necromancer prisoner and drag him back into the village he threatened in chains, for raising morale among the peasants. Extra pay, anyone? :) Tactical retreats, both on the strategic and tactical map, enemies not even engaging you when you’re clearly stronger, and the possibility of different outcomes than just “win” or “dead” for both sides are what I want to see as well.
I’d actually love to see a feature of “knocking out” combattants in the game. It would work for both sides, but take for example a Battle Brother being hit in the head with a mace by an orc. Maybe his helmet holds, he isn’t dead, but he’s out cold for the rest of the combat. If his side wins, they’ll just wake him back up and give his helmet to the blacksmith, but if not there’s still more than just “dead”: a Brother could be coup-de-grace’d by the opposition after the combat when they notice him still being alive, he could be taken prisoner, be simply declared “missing” after both sides quit the field of battle (new possibilities arise from that unclear state!), or even be mistaken for a corpse by the enemy and be able to flee after the battle and possibly re-join you two days later on. I think that would really make the combat in Battle Brothers three-dimensional, so to say. A mechanism where combattants are too wounded to carry on fighting, but are not dead (and not exactly unconscious), could probably be subsumed under this umbrella, so basically an enhanced “casualty mechanic”, where casualty doesn’t equal dead (just as it doesn’t in real life).