I like the base they've built, so it would be nice to see that fixed up, and the very rough parts worked on, such as kingdom diplomacy being incredibly barebones. Since this kind of game is often all about mods, I'd like to see a really solid base of mechanics. As it stands now, some of the basic mechanics I'd like to see finished/polished to make it properly usable:
Battles. I think the collision issues both in terms of weapons and unit size are problematic. Fighting in formation is problematic. Archer not firing in formations. Cavalry only really doing the damage of their weapon instead of really devastating lines. Etcetc. I think it should feel a lot closer to first person Total War than it does.
Economy. I think they got it right by focusing on trade and production (caravans and workshops). That's really general and interacts well with the map, resources being tied to locations, the stuff on market being decided by marketplaces, etc. I'd like more interactions/control over caravans so that they are more interesting to set up well, and especially so that protecting them is more specific protect this route or area and not a global "don't be at war much, then dudes kill more caravans that roam randomly" thing.
Land and ownership. It's great how it interacts with everything else, but I think again there should be more agency. For instance, set up a food caravan to provide lots of food. Set up food production for extra food if you need it, but it might not be enough in an infertile area or with raiding. Right now it's spam small upgrades, not much interaction. I like that gaining land can be about diplomacy and politics.
Diplomacy/politics. This is extremely bare-bones and should be expanded so you can do the basics: Make deals around alliances, peace, war, trade and ownership.
Relation system. I like the idea, but atm it's mostly frustrating or a grind. I think it's important that it integrates with the rest of the systems, such as battles and trade (it does a little now, but more). I think relations are best when they correspond to the player liking the person eg a character you like to fight with or a recruiter close to your main base that has good troops, or correspond to likelihood of benefitting one-another in the strategic sense, for instance if you were essential in capturing a city for another clan leader, or you aid an ally in defending their home.
Lineage, family, characters. I think Crusader Kings 2 has a lot of inspiring things here. Making the game more character-driven adds a whole other source of appeal that interacts well with relation system, diplomacy, battle stories (rescuing a famous warrior and he becomes an ally!).
I think you get the idea. They're touching on many small things that are quite general, and they've made the pieces for things to interact. This is wonderful. I don't envy them their balancing challenge, but everything interacting will for me be the heart and soul of Mount and Blade.
Sidenote, I think the output of the smithing system is cool. Make the weapon you want both in terms of looks and functionality. Not sure the way the grind is set up is great atm.