I've practically designed a system for that and posted here years ago, totally ignored.
The core of it would be that AI would take hefty casualties every siege within Auto-Calc, and personalities of leading lords would define if they'd take the risk of assaulting or not. This way giving room for real siege warfare where it's all about starving and demoralizing the defenders, while the defenders are actually trying to hold-out for backup armies.
If such a thing would be deployed alongside a decent diplomatic system extension, ownership of fiefs would determine war results and sometimes be the stake for peace treaties, as a Ransom for Royals (rarely), and if won either by assault or outlasting the enemy + not losing to the enemy's main army when it came to the rescue. Add in a chance of the lord that holds de fief deflecting as to not losing his lands, and expanding the siege mechanic to support realistic sieges, and there you have it. The interesting part is that it can fully take account of the noble's personality to determine how both sides handle it. Surrender+Deflecting, Surrender, Holdout as long as possible at the cost of garrison starvation, Holdout until supplies are over, than the Sally Out, flee to avoid capture or staying. If done right it'd add a lot of depth to the game because we'd start to pay attention to lords personalities for real. Adding a strong "de jure" system would help the AI negotiate peace treaties for extensive costly wars, as in they simply give the fief back to the rightful owner and bam, war's over.
Possible outcomes could be incredible. IE:
The guy's stubborn, holds out until the last straw, but is also calculating and simply deflects at the last minute, surrendering to the king but keeping their fiefs.
The guy's daring, honorable and practical, so he sallies out as soon as starvation comes in a heroic last-stand.
The guy's cruel, greedy and calculating, starts cannibalizing his own men allowing for them to outlast the attackers in food and supplies
The guy's cruel and calculating, he simply surrenders as soon as the siege starts if his relations with their liege are low.
so on so forth. With that and giving mini-missions on scene to take control of the water source and poison it, stop sneaking caravans of supplies from getting inside, sallying out with minor garrison numbers to skirmish and protect incoming supplies, use long distance siege equipment to lower defenders morale (indefensible from within the fort, but doesn't cause any dmg to walls), trying to negotiate and flip garrison soldiers to open the gates, etc. That'd be beyond perfection.
But to pull such a thing they'd need to fix the army food consumption on sieges + add a ton of intricate scenes that involve the fief but isn't an assault. And likely give a existing stance where assaults are diamond jewels, just make it 30% assaullts 70% wait-out sieges for the AI and we'd be golden. Taking into account that everytime an assault's succesful, it'd still ravage the ranks of the attacking army (something like losing 70% of the troops on average)