I like the idea of battles that end with survivors on both sides, including morale and fatigue mechanics. Why? This brings up the issue of RESERVES. Consider one of the big problems with 4X games, the 'Stack of Doom (SoD).
Round 1: SoD attacks stack A and wins. Stack A retreats, and stack B fights next round.
Round 2: SoD wins against stack B, but less so because they are tired. Reinforcements reach stack A.
Round 3: SoD loses against stack A, because the troops are ALL tired, and A has had time to rest and resupply.
Round 4: SoD breaks in battle with stack B, and flees because it knows stack A is resting this turn.
I'd like to see this sort of thing allow inferior numbers to 'guerilla war' a numerically superior force. Of course, it just forces 'stack cohesion' where the new SoD is actually three stacks that stay close together. Gr. I still like the idea better, if we're getting together battles that measure in the thousands.
Maybe a slider bar in the cultural panel, that lets you control when your leaders are AUTHORIZED to withdraw from the field? Or maybe certain leaders grant less of a combat bonus, but their troops suffer less morale and fatigue loss?
And let's not forget the Persian Archers, who 'lost' battles daily (ran away) but returned every day to kill five men for every one they lost.
And to steal an idea from another thread, maybe I see a battle isn't winnable, and my mobile units (Bear Riders and Gryphon Knights) have orders to withdraw, leaving the peasant rabble to delay the attacker?
Note: I'm not saying all battles should be like this. If 300 Spartans arrive at the Haut Gates and defend versus 100 times their number, they just aren't going to be able to hold out for more than a day... okay, bad example. But if an army is trained in that terrain, they'll survive better. Never fight elves in the woods - it only encourages them.