I feel like the map system, which is the glue that keeps the combat and everything else together, is and has been far too simplistic. It would be easy to mass generate a random map, add more terrain features and make the world map more than basically a test of movement speed optimization between armies. There need to be way more sites that aren't cities or castles. It's like playing on a giant RISK map, that's basically featureless for all intents and purposes.
Secondly I'd like to see more of what they did in Warband, the developing of your own kingdom. The ability to found towns (and wipe them off the map completely.) I want the ability to own a piece of the world ala TES games, without having to start a war to do so.
The character system could use new skills. So many of them are niche skills that you feel wasteful spending points on. The conflict between avatar combat development and leadership abilities could be made a lot more interesting from the avatar side. Because your reasons for investing in your HP, power strike, that stuff, go away as your army starts to do the bulk of combat for you.
Mercantilism could also be taken a little more seriously. From what I've messed with it, there's no real depth to it. Merchant quest lines, setting up shops, trading partner or merchant co-ops. Those would make it a viable game play path versus mercenary/lord/king.
Lastly, some actual plot would be nice. Letting the AI beat up on itself in an endless cycle doesn't give the world any personality. Sure some lieges have some traits, but you can't really distinguish the Swadian King from the Vaegir King. Plot lines between kingdoms, that actually drive the cycle of warfare, would be great. Even if after 20 hours you see it for what it is, it's more than "Welp, time to go to war, because that's what we do!" The Usurper Quests are there in Warband....but there more like in points for you to start a war without being responsible for it. Everyone has a Usurper so it's not like the quests are unique. And they're not top notch writing either.