I would like more imagination than we saw in Warband for sieges as a whole. While many people enthuse about doing real time damage with siege weapons in the battle map, which is admittedly fun, it could distort the balance between castles and siege weapons from a fun and immersion point of view I think. Huge stone walls being demolished in minutes makes a mockery of their defensive value. If in singleplayer we can break down a wall with a catapult in a few minutes in the siege scene, then all the defender can do is leave his men on the wall to be battered to death or wait for it to fall, and then suffer a quick death as the attackers flood in. That won't be so bad if building catapults requires a skilled engineer and takes plenty of time, but I would still like to see more from sieges- such as the defenders retreating through the streets or to the keep, or having different scenes before a traditional assault takes place.
For example, while siege weapons are being built or the besiegers are waiting for the defenders to starve, there could be randomly triggered or player initiated events such as the enemy trying to sally out and destroy your engines. You would get a message that this was happening, then enter a scene (probably at night) where your catapults and towers were being attacked with torches by the defenders, and you have to fight them off (perhaps dressed without your armour, having left your tent in a hurry, and with only a few nearby guards and companions to help).
The player could try and sneak into the castle; this might be through a companion who had sneaked in before the siege and now sends a message that he will lower a rope at midnight. You then choose a small force of shieldless men (so as to be nimble and quiet) to scale the wall with you and try to take control of the gatehouse; guards may spot you if you bring too many troops, or if you are defeated you will be captured and then could be used to get your army to leave, or you might decide to retreat if the gatehouse is too well defended- or you might succeed. In the case of success you would then leave the scene, and the next scene would have you respawn in the gatehouse while your main army rushes in below you- horns blow in the castle, waking up the defenders, who you have to fight off for a minute or so until your main army comes up to the battlements to help you. After that you stay in the scene and chase the defenders to the keep, fighting through the streets if it is in a town, and then using small portable rams or axes to break into the keep.