A quick, top-of-my-head list of features that will be in Bannerlord (and weren't in Warband), but don't exist in any mod I'm aware of:
- Weapon crafting and customization.
- Procedural quest generation based on the ever-shifting circumstances of the world during play.
- The entire clan system. Just the whole thing.
- Permadeath and heirs created through a genetics system utilizing facekeys.
- Upgradable defenses for castles and cities.
- Overhauled recruitment system derived from relationships formed with important NPCs.
- Formation-based command and deployment.
- Tiered AI, informed partially by the general's personality, allowing individual soldiers to exercise their own AI in the framework of larger formation-driven considerations, which in turn are executed within army-level strategies.
- Formation tactics (shield walls and so forth).
- Item necessities for powerful troop promotions.
- Persistent, activity-based economy that every NPC takes part in.
- The need for NPCs to recruit from the same pools of troops that you do.
- On-map campaign level representation of siege camps, town facilities, etc.
- Physics-based combat and damage calculation.
- Cloth and mane physics!
- A true day/night cycle with a physical sun, and global illumination to make it extra pretty.
- Keep battles in sieges, and enemies that will withdraw to inner defenses when outer positions are overrun.
- Conversational animations, which just plain look better, it's not necessarily a "feature".
- Siege engines. There are two mods I know of that did this. One was multiplayer only, and the other was limited in that it was only present in certain scenes, and always present in those scenes.
- Physically grounded production facilities that you have to compete with AI rivals for ownership of.
- Fief construction projects, allowing us to improve our fiefs and specialize their roles in our kingdom.
- Governor appointments.
- Execution mechanic.
- Crime and punishment tracking, adding immense depth to bandit gameplay.
- Overhauled village interactions including changes to village raids, the ability to forcefully recruit villagers, and the ability to do all this from caravans and roaming villager bands as well (all of which interacts with the aforementioned crime and punishment system).
- Campaign logistics, including...
- Food sharing
- The ability to butcher mounts and pack animals in case of emergency
- Forage party mechanics (which also adds a realistic depth to the campaign layer in that you can ambush forage parties to cut off a large army's food supply)
- Scorched earth tactics
- Improved siege mechanics that make investment matter while also tying into the new provisioning mechanics, making food all important for defending armies.
- Mechanics for breaking through sieges to get inside besieged castles, or attacking besieging armies from outside the walls in the actual siege scene.
- Personality and reputation systems for the character level, overhauling the way we interact and form bonds (or rivalries) with AI lords.
- A full-on persuasion system that interacts with that and introduces the concept of courtly negotiations.
And this doesn't even touch things I'm not sure will be in the game, like horse breeding, bandit gang mechanics (something I heard about but haven't seen in dev blogs for myself), and an endless supply of potential companions via random generation.