(I didn't read any of the posts before this except the first to start off so my ideas may be repeats)
I was thinking along the lines of the small little things that make a game like warband or this a lot more fun. Since I'm pretty sure you guys got the main game mechanics outlined or done my idea is small things, stuff like pulling an arrow out of your body, cutting all the arrows off your shield with your sword, hitting someone with your crossbow to stun them and then be able to pull out a sword, stabbing someone in melee with an arrow when they get to close (or throwing the arrow), throwing your sword, using a bandage/cloth on wounds, eating and drinking (its a small thing for a game like this i would think), wild animals, trained animals (other than horses and donkeys), being able to place deployable pavise shields, weapons or shields getting knocked out of your hand or destroyed, cries of surrender and cages/prisons to put prisoners in, random weapons around the battlefield (not from dead people), random ai warriors on battlefield that help or try to kill you, shops to buy weapons ammo or other stuff on the battlefield, random events (storms, branches falling, building falling over, etc.), and other little things that you might come up with will make the game more enjoyable and random which will always keep the player guessing. That list right there was what i came up with in 15 mins although i may have repeated some things that other people said i hope it helps if you guys use it.