Sounds promising, but more info about the The Big Update™ would be nice! We barely know anything about it ;).

Sure! The Big Update is primarily focused on abilities, not units.
It's possible for garrisons to be more exciting than a bunch of guys sitting in the settlement. I hope to be able to tap into the potential design space that's there.

It also wants to bring more relevance to the garrison/wall buildings. Currently with Reporting For Duty, there's less incentive to build them because you can already grow the garrison by other buildings. However, simply making those buildings stronger to make them more attractive isn't my ideal solution. That can feel akin to creating another problem by fixing one. A large challenge for that is compatibility with John Austin's Tailored Faction Garrisons - which is actually a unique building chain for each different faction (it's actually a beast of a mod). So ultimately, we'll see what happens.

I definitely have some suggestions!

I will start by saying that there is one race which ATM doesn't really need any additional abilities for siege defense and it's Skaven. The combination of Warp Bomb, Menace From Below, Warp Lightning and very strong towers already make Skaven cities (even tier 3 minor settlements) one of the easiest to defend... and also the most unique ones, too. Therefore, Skaven should absolutely be at the bottom of any to-do list as far as siege abilities are concerned.

VAMPIRE COUNTS

IMO Vampire Counts deserve some focus. With no artillery, no ranged units and towers that aren't super impressive, their standard garrisons are at a big disadvantage during siege defense when compared with other races. Potential improvements:
1. Zombie and/or Fellbat summon ability similar to Skaven's Menace From Below so that the siege defenders can disrupt artillery and ranged units.
2. Invocation of Nekhek ability for major settlements. No WoM cost, just some free healing on a cooldown. High tier walls would increase the maximum number of ability uses and/or lower cooldown. TBH with reasonably high cooldown the maximum number of uses isn't even really necessary, since healing caps exist.
3. There is a lore blurb about Castle Drakenhof being built on the former lair of a large dragon. How about permanent summonable terrorgheist for tier 5 Castle Drakenhof? :) It's cool and you don't have to worry about 20 unit garrison limit.
4. Random suggestion, not really connected to siege abilities in any way: consider increasing the presence of Mortis Engine and/or Corpse Cart (Unholy Lodestone) units in standard garrisons for tier 4/5 cities. They can potentially be very useful for fights in gate blobs and choke-points.

DWARFS

Dwarfs, while already quite decent at defending in-game, get too easily outshooted by siege attackers IMO. In lore their strongholds are hard nuts to crack and they usually enjoy the artillery advantage. Therefore, as a standard generic ability for siege defense they should get some sort of Vampire Coast-style artillery bombardment on cooldown. High tier walls would increase the maximum number of ability uses.

Karaz-a-Karak could get some unique abilities. There are lore blurbs about steam engines which can cause avalanches. This could be a potential strong bombardment rock drop ability for tier 5 Karaz-a-Karak (there are already spells like that in-game). Also, the gyro landmark could provide a permanent summonable gyrobomber. Again, fun stuff and you don't have to worry about the garrison limit.

LIZARDMEN

I think that Shield of the Old Ones as a standard generic ability could work quite well for the Lizardmen. Very useful in gate blob fights and in choke-points. Good synergy with tanky Lizardmen units as well. High tier walls would increase the maximum number of ability uses, while high tier geomantic web building would decrease the cooldown between uses.

Additionally, I think that Burning Alignment as an ability for tier 5 Hexoatl would be fun to use. It's potentially devastating in a siege defense situation, but capturing maxed out Hexoatl shouldn't be easy. Itza's Vaults of the Old Ones landmark could provide a similar ability as well.

IMO Temple of Mists landmark is underutilized. I think that it should provide some additional low-tier summons to give that "emerging from the mists" feeling. Maybe summonable Chameleon Skinks? Also, an ability that would lower the reload rate, range and accuracy of enemy missile units could be useful as well.

THE EMPIRE

I think that generic ammo-replenishment ability would be very useful for the Empire during siege defense. It would have great synergy with units like mortars and handgunners. As usual, high tier walls would increase the maximum number of ability uses.

Altdorf could get the unique ability that would make units unbreakable in an area of effect. Good to give that capital defense/last stand feeling. I don't think that this ability should have a maximum number of uses, just a cooldown. The higher the city's tier, the lower the cooldown.

Nuln definitely deserves some artillery bombardment ability. Maybe even two abilities? One Queen Bess-style (big boom in an area) and one more like a wind/breath spell. Could be tied to Nuln's landmark.

These are great ideas cybvep, thank you!

More ideas for consideration:

1. Permanent Phoenix summon and Fiery Convocation ability on cooldown for Shrine of Asuryan's landmark (HE version). Phoenix summon could be available provincewide to represent their mobility. As for Shrine of Asuryan itself, I think that an ability on cooldown that gives ward save and imbues units with magical and flame attacks in an AOE would be appropriate. Also, a permanent map-wide debuff that increases enemy's miscast chance by 100%. All this should represent the magical defences of SoA. There is nothing truly special about this place on Vanilla, which is sad.
2. AOE slow ability for Galleon' Graveyard on cooldown. The higher the settlement's tier, the lower the cooldown. Higher tiers could also give summons as abililites (standard ones, i.e. non-permanent with maximum number of uses ). Tier 3 could give an ability to summon zombie gunnery mobs, while tier 5 could give an ability to summon Necrofex Collosus. All this to represent what Galleon's Graveyard is supposed to be - a magical whirlpool that draws the dead sailors and shipwrecks.

Some small random stuff:
1. GS Boss Shack should add Goblin Big Boss to the garrison.
2. EMP Tavern should add the Witch Hunter to the garrison.
3. DW Hostelry building should add Rangers to the garrison. The lvl 4 version should add Bugman's Rangers.

Hey cybvep. Thanks for those 3 suggestions. Point by point:
1. Let me look at it. (Update: No. This would place two Big Boss heroes in the garrison.)
2. In the middle of me typing out why I haven't done this, I just realized a way to do what I need for it. Sweet. (Update: Done!)
3. No.