Most of us agree that the current state of sieges in WH3 is the worst in the entire series. So this is what should be done in order to make sieges epic:
The walls need more HP. Only siege engines/artillery should be able to damage it. Not all LLs have siege attack. Infantry takes a lot of time to destroy the gates or the wall. Walls have a anti-magic layer on both sides of it so that the casters cannot one click kill all the blobbed units in front of the gates for example. If you destroy the walls, the anti-magic barrier in that area is destroyed, meaning that both attacker and defender can use AOE spells there. Walls should be wider, and units can only get on it through doors, and not from everywhere, you click. The walls are wider, but the units can fall down the walls. You can also use your infantry units to kick down the ladders.
The three-layered settlement as in Shogun 2 where you have the citadel in the middle. Each wall of defense broken gives + 10 leadership to the attacker and -10 to the defender. Some races like dwarfs should be able to fight till the death, with no leadership penalty in their cities. You capture the main citadel, and the city has fallen. No capture points are scattered around the map, just the leadership buff/penalty for destroyed layers of walls. Only 1 capture point at the citadel means that the AI would work more easily.
A bit smaller cities (but not too small, most of the settlements are ghost towns and are unable to defend with 20 units only)
The main problem is that ladders/siege towers/battering rams is 20 years old formula and it simply does not work anymore. Therefore, CA should make unique siege engines and unique siege defenses.
Ladders, battering rams and siege towers all take turns to build. The same thing goes for unique race-themed siege engines and unique siege defenses. Some races have better siege engines as the attackers, while others have better siege defenses. The problem of sieges now is that the attacker is heavily disadvantaged, because of walls, walled towers, poppables. Now, battering rams are useless because some units can destroy the gates before you bring the battering rams. Battering rams have more HP and cause a lot of damage to the gates (that have a lot of HP). Ladders can be kicked off the walls.
For example, skaven and dwarfs should be able to sap walls via underground tunnels. Or they can just,make tunnels on the other side of the cities. Or you can bring forth Dwarf Zeppelin to bombard the walls, or some gobbo catapults that catapult + parachute gobbos inside the city. Or Bretonnian trebuchet that spread the plague after being used during the siege. Siege engines should be BIG, like LOTR Gondor style big. Without them being big, sieges will never be epic spectacles.
All races have burning oil at the gates. Unique siege defenses are also race themed - for example, nurgle has walls with eyes that protect them from stealth units capturing artillery or gates. Nurgle also has corrosive bile at the walls. They also have living walls with eyes + living towers - a bunch of tentacles with big range that fight in melee. Tomb Kings have Archimedes' Death ray that burns down everything on its way. WoC have molten lava around cities as the means of defense, while human factions have water ditches that must be filled with sand in order to cross. Vampires have a passive ability to make zombies out of the fallen enemies' units. Lizardmen have jungle rot that debuffs the enemy, while Kislev has some frost debuff.
So depending on the city you are fighting in, there are advantages and disadvantages, so some races might find it easy to besiege X race's city, while the other is struggling. Or vice versa. But the main point is, cities should give you a feeling that you are fighting in a certain race's territory which has consequences for both sides.
Poppable towers are removed from the game!!!! No need to elaborate on that.
Defensive supplies mechanic is expanded. You have a lot more options when it comes to defenses. You can spend supplies to stop the attrition (by spending it as food for your troops), or repair walls and towers, or upgrade towers on walls, or have access to WALLED ARTILLERY, stronger walls/gates, stronger burning oil, water/lava ditch around walls, spend it on ammo for your ranged units. So basically, you have a lot of choices to choose from, but with limited amounts of defensive supplies, you need to spend your supplies carefully, depending on what your aim is during the siege. Traps are pre-deployable, and there is around 8 of them. They can even be made to be race-themed to make races feel more unique. Example: Bretonnian archers have anti-cavalry stakes. Flaming balls are a thing for ambushes. Each race has a hero or ll that gives you access to certain siege defenses/siege engines. They also give you bonus defensive supplies, or supplies replenishment for the settlement.
Another thing would be sally forth night battles . So basically, as the attacker takes for instance 3 turns to build some unique siege engine, you can sally forth and try to sabotage his new big guns. You can either destroy the damn thing, or let's say damage it 33% (which is equal to 1 more turn for the siege engine to be built). These missions are being carried out with cavalry or strealth units during the night battles, and the attacker is not fully prepared, with most of his units scattered around. Same thing can be done to destroy some ranged units of the attacker.If you destroy the siege engine and come back to the city gates, you are victorious. if you fail to do so, the attacker can instantly attack your city in retaliation. You literally choose should you attack the city or call it a day and continue with siege attrition.
Another main thing is stealth units being able to climb walls. Their role in sieges would be to open the city gates without the defender even knowing it, or to destroy siege defenses, or even use walled artillery against the defender. Some units like spiders climb walls, while for example death runners have ninja hooks that make them across. In order to defend yourself from stealth units, you have to place a unit on wall, or you have to make unique siege defense (aka watchtower) that only gives you line of sight against stealth units.
Regarding the minor sieges, I think that land battles is a terrible idea. The distance between settlements should be bigger, and therefore, the map needs to be stretched. The main frustration with sieges now is that when you are attacker. Momentum mechanic simply doesn't not work as intended and cannot compensate for all the power you get from poppables, walls and walled towers.
The main approach should be that neither side has the advantage, and that the win/lose ratio should be 50%-50% for the attacker/defender. Now, it is heavily favoring the defender, like 80%. CA should just look at siege of Gondor / Minas Tirith / Helm's deep. Or just go to good old Stronghold Crusader and see how many traps and siege engines you have there. Kingdom of Heaven can be another great reference.