I'll give you a brief summary of what i've imagined and then some reasoning after.
After a battle, the winner places his wounded troops in available hospital space, and enemy troops in prisoner camps. These two parties can then release prisoners in exchange for cease-fire or peace deal!
I don't think it will require much effort, one problem is how to tell which troops become wounded and which die, but i would love to see large and long wars ending in stalemate so that the released prisoner troops can be put on new fronts, maybe with the former enemies ("oh shit, we both have 500 prisoners? you know..."). It kinda allows players to win losing wars and troops become less of a number (maybe the last is a bad thing considering the aesthetic)