My personal wishlist for M&B consists of the following:
Better sieges (Multiple paths of assault, siege weaponry, defense weaponry, options while starving out the garrison to do things like fire rotting corpses/sacks of filth into the castle to try and induce sickness in the defenders, sap the walls, try to send one or two troops in at night to raise the front gate, etc)
Sea travel and sea battles. Pretty self-explanatory, maybe even allow assaults from the sea when sieging shoreline cities.
Thrusting polearms (2H and 1H with shield) not useless compared to every other weapon. Multiple directions of thrusting so the AI doesn't just hold down the entire time and casually stroll into spearman blocks without a single casualty. Rhodok infantry really suffer thanks to this. In Warband, I can kill up to mid-tier units with a dagger that can't block more easily then I can kill them with a spear and shield just because the dagger has more than one attack.
City/Village/Castle Growth. As a lord, I'd like to be able to invest heavily in my property, and see it grow accordingly. Being able to add additional farms to villages to up productivity, a garrison that can defend itself against bandits or generate a patrol that can take out small enemy forces, institute weekly/monthly militia training at the cost of some profit that results in my recruitments turning up low-tier soldiers instead of fresh recruits reliably and makes the garrison/patrols stronger (instead of the current high reputation, random chance system with no self-defense capabilities right now). Similar actions for castles and cities. I'd ask that the locations physically change with your upgrades, but that might take way more resources then necessary, so holding off on that would be completely understandable.
Not stealth. The focus of Warband has always been on commanding troops and raising armies. Certainly, players fighting alongside their army is a core part of the game, which is what draws me to it over pure RTS games. However, a stealth system would be meaningless in regards to armies, given you can already maneuver out of the range and sight of enemy armies so long as you invest in Pathfinding, Spotting, and tracking. And no commander would be dumb enough to send themselves in as reconaissance or scouting, since getting captured or killed would mean the immediate loss of the command structure for their troops, as well as a major morale boost for the enemy. Adding stealth for the sake of "individual" activities like pickpocketing or assassinating would just result in more time being taken away from the core gameplay: recruiting troops to build armies, conquering kingdoms, and building up an empire, as well as unnecessarily complicating combat. I also refer to jacobhind's post here for further support
This combined with a stealthy /army mechanic would allow for resourceful commanders to take on larger forces if played correctly.
The problem is that you've got to balance it so that the AI does it as well, which (in games) usually means an annoying popup saying you've lost x amount of troops to an unavoidable logistics sabotage. It's one of the reasons why I hate the agents in the total war series so much; they're unavoidable and you/the AI can just churn them out and deal nation-destroying damage for practically zero risk.
Given that in warband (and in the middle ages) most armies wouldn't have supply lines that were very long, a scorched-earth mechanic would make more sense. There are plenty of occasions when the enemy would be able to utterly screw me over by siphoning off the food in nearby villages (even with the ability to feed an entire army for weeks from an impoverished village's dregs). At least that way, there'd be a huge risk involved with sending the regional harvest up in flames.
Battlefield awareness of the A.I. It'd be good if we could maneuver using terrain in the field to slip around the enemy without them rotating in place and being hyperaware of our location even when we're completely out of sight in an area that provides multiple hiding places like hills or ravines. This is the one form of stealth I'd be okay with, since it's less about "You are a super awesome ninja that can kill entire armies by hiding behind trees and popping out every so often", and more about good combat tactics and positioning your forces.
Bigger battle sizes, while preserving the size slider. I sympathize with the people who want giant battles, and it would certainly allow for more engrossing combat. At the same time, one of the draws of Warband is that it can run on "toasters", so to speak. Keeping the size slider would allow both parties to be satisfied.
I am ambivalent about dual wielding. As long as its weaknesses are properly represented in that you don't have the same strength behind your blows as holding a weapon with two hands, you can't defend yourself from arrows like you could with a shield, and blocking attacks takes more precision and maneuvering then with a shield, making it more troublesome to use in anything other then small scale battles against mostly close-combat opponents, I'd be fine with it. But given how most games (especially Skyrim, the game most people who suggest stealth and dual-wielding seem to want Bannerlord to turn into) make people who fight with two weapons into some sort of superhuman tornado of blades, there's a sense of unease about the entire thing.
I don't give a **** about romances, and to be frank, romances draw writers away from stuff like lore, backgrounds, and quest plotlines just so people can get their jerk on. The current romance system is fine as it is, especially since it's fairly close to what feudal courting was like anyways. Maybe named companions can get more intimate romance systems, but I don't really see how making them your bed buddy would provide any benefits in battle or commanding armies that would counter the drain of time and resources from core gameplay.