I think it's fairly rational to say that siege battles will not get a revamp. At least not the layout of each map, because the time it would take to change all the maps would be a ridiculously time-consuming undertaking. What I feel would enhance sieges isn't really the layout, but the way each siege feels depending on the faction that you're attacking.
Every faction has a unique feel to them on the campaign and in land battles, but sieges somehow always play out the same way and feel too similar to each other.
So instead of just changing siege maps, they could add new mechanics for each race to make sieges feel different and more exciting.
I've already mentioned in a previous post that having civilians in siege battles could make interesting scenarios were as a defender you have to protect the civilians and as an attacker you can kill as many as possible to lower the enemies moral/give them penalties on the campaign map.
But here I will talk about some ideas that I had which could make each faction feel different when defending in a siege battle. So here it goes:
Dwarfs - Sieging a Karak/Dwarfen stronghold should not be easy. The Dwarfs could gain powerful defensive buffs and spaces on their walls to put siege engines on. This way CA doesn't have to change the entire layout of all maps, all they gotta do is just change a wall tile-set.
By that I mean, if now there's only 1 model for dwarfen walls, they could add a second variety that is more roundly shaped on which the dwarfs can place their siege weapons.
Or they just make a different set that has cannons mounted on different levels of the wall. Setting ladders on those walls would be a bad idea cause the cannons could just blow them off. As an alternative to cannons you could also place cauldrons full of burning oil to pour down on enemies who try to climb up. Dwarfen walls and gates should have runes on them which make them much more resistant to damage. Maybe a wall variety that adds runes that cause damage over time to undead that are near the wall? A rune system with different buffs for the Dawi could be REALLY cool. A rune dedicated to a specific god which you can choose in the campaign that will give your armies on them unique benefits.
Dwarfs can also have an area within the capture point where they could grab ammunition for their ranged and artillery units, but that ammunition should be finite. Perhaps it could get upgraded with certain buildings or research?
Vampire Counts/Vampire Coast - Attacking vampires could make skeletal arms reach out of the walls to pull down soldiers who're climbing up the ladders, or they could have stone gargoyles which periodically cause a shockwave that causes terror. Vampire Counts can also periodically get zombies as reinforcements outside/inside the city, to give this sense of numberless masses that will overwhelm the attackers unless they defeat the vampire lord/necromancer.
Tomb Kings - Attacking a tomb king settlement should have fragile liche priests assigned to x parts of the wall which give an AoE heal and resurrection to undead minions. The attacker would have to focus these lich priests first otherwise they'd face tides of skellies. Similar to the vampire counts, the Tomb Kings will get constructs as reinforcements after a longer time period.
Human factions - I think humans can get the same "cannons-on-walls" and ammunition treatment as the dwarfs, But the only human faction that should really stand out in terms of siege defences is Middenland, cause of their great cannon. Kislev could use a snowstorm army ability which blows people off of the edge of the walls and from ladders, or they can freeze siege engines for a short time on their track.
Greenskins - Greenskins gotta have some sort of "enrage" mechanic to make it dangerous if you drag out a siege on them. The longer the siege takes the more fightiness they gain. It could work as a sort of bar like the DE or TK have. With each stage of fightiness that you pass, you gain some buffs such as leadership, extra charge speed and charge rating. Once they reach max fightiness, they will get a large number of reinforcements (5-7 regiments) with units from any tier. units. The idea behind this is that the longer the fight takes the more other greenskins and monsters in the area are attracted to the fighting, just like described in the lore with certain beasts that the greenskins fight alongside with.
High Elves - HE can have a sort of ritual mechanic. If they get besieged they can have a mage within the city that starts channelling a ritual. The ritual grants increased winds of magic and recharge rate (which can be increased be certain buildings) for not only the HE but also their enemies, but the enemies have increased miscast chances (can be increased via buildings), At higher settlement/wall levels they can also have army abilities which can cast a mist in an area to make their forces unattackable by enemy units and untargetable by spells (except for vortex, bombardment and cone spells) for a small duration, enough to pull them out of a sticky situation. Enemies can also get slowed by that same mist so it can also be a good way to kite enemies.
HE should be about overwhelming their foes with magic and using hit and run tactics.
If the mage is killed, the HE lose these abilities.
Dark Elves - DE could have an army ability (let's call it "Gaze of Khaine" for now) which acts as an AoE buff that can be moved by the player for x seconds before it disappears. The AoE should be small enough to only affect unit/regiment. It will grant a large amount of offensive stat buffs and grant frenzy and enrage, and this can happen to both enemy or ally.
You can use this army ability to either buff a single powerful unit to make them into a killing machine or shift it to an area where you want an enemy to lose control at the cost of also becoming dangerous. That way you can at least
If the AI cannot handle moving an AoE spell around then it could just be a point and select type of ability like Soul Leech.
Along with their murderous prowess I think the DE would be in a relatively good spot.
Wood Elves - WE should feel like a pain in the ass to beat when you fight them in the forests of Athel Loren. In a WE siege, their ranged units get the snipe trait as long as they are in forest areas and all WE units gain movement speed within said forests, because they know every nook and cranny in their own region. They should move fast within the shadows and strike fast and hard before they return to attack from somewhere else.
As a cool army ability, they can grow trees in an AoE which will give them the same buffs as previously mentioned and become a detriment to their foes if they go in them because they'll get the regular forest penalties.
Skaven - Similar to the dwarfs, they should have unique wall assets that either let them mount their weapon teams/artillery on it or as an alternative they can have a wall asset that has weapons that shoot warp bullets or warp lightning at their approaching enemies. Skaven should also have an area where they can grab ammunition (as already mentioned, the ammunition should be finite but increasable via buildings/research).
Near the capture point the Skaven can have a powerful machine which needs to get charged up for a set amount of time. If the Skaven aren't defeated before the timer runs out, they can unleash the device which will shoot arcs of warp lightning in random areas within the city and walls, which will cause horrendous damage. They might either completely break the enemy or themselves, but this sort of self sabotage and random chance is what makes Skaven into what they are. A second variety of this device can be one that can fire concentrated blasts of warpfire into an area, but the fires are so volatile that they can spread quickly and turn the entire city into a giant BBQ.
It would feel very skavenlike to have such firepower and cause near-mindless destruction.
Lizardmen - If a Lizardmen temple-city is attacked, their Slann awakens from their slumber to aid their forces. This would also be a neat way of seeing more Slanns throughout a campaign when heading to Lustria/the Southland Jungles. The Slann is situated in the capture point and can create a dome in a large area around it that will block projectiles for a duration. Like what we see Teclis do in the WH2 cinematic, only larger. Kinda like how Kroak held a dome around Itza for 10 years to prevent daemons from getting in, but this would have to be a really toned down shield.
To make it more interesting, apart from the dome that I previously mentioned, you can have the choice of casting another version which causes severe damage to enemies who get in. Imagine daemons squeezing through the dome and getting melted in seconds.
Apart from creating a barrier, the Slann can also repair destroyed walls with their telekinetic powers. And again, if the Slann dies, the lizardmen will lose these abilities, UNLESS another slann from the defending army is there to help.
And of course, the Lizardmen are a very magic focused faction. So due to the Slann, the Lizardmen will gain a large WoM pool and WoM recharge rate and if a Rite summoned Slann joins the battle, these WoM buffs are increased.
Norsca - When attacking a Norscan stronghold, they can call upon an AI controlled Champion of the dark gods after a certain amount of time, if the power bar is against their favour. Imagine holding your ground for 8 minutes and racking up kills, and you slowly start feeling your forces losing momentum, when suddenly Valkia the Bloody swoops down after deeming you worthy and shifting the tide of battle.
And finally, certain units could behave differently when in a siege. I think ethereal units should have the option of just passing through walls if you click behind a wall. And spider-riders and potential future units like Flayerkin should should be able to climb walls without having to line up for a ladder.
And these were just some ideas I had fun with folks. What do ya'll think?