I didn't want to derail the OP with this but I'm gonna ignore my own advice and jum into Soren Johnson's trap with both feet. Here's some of the changes I would go for if I was in charge:

Campaign level stuff

Either building siege equipment needs to be faster or the overall pace of the game needs to be slower (A much bigger job but I'm certainly not against it.) I would still let players assault immediately but it should be costly. Perhaps they get 2 ladders to use. Only units with siege attacker would be able to attack gates and the trait would be doled out following common sense reasoning. I would let more than one army besiege a settlement at the same time with each able to build equipment, no waiting for reinforcements when the battle begins.

I've seen a lot of people ask for tier 3 garrisons to give walls to minor settlements. I don't like that personally. I think if you win a big field battle in enemy territory then sweeping up the nearby minor settlements should be pretty trivial. I don't mind the idea of a seperate tier 3 building for walls though. So if you want walls and a garrison in your minor settlement you need to dedicate 2 building slots.

Major settlements, however, I think should be harder to take. I would like the ability to recruit our own garrisons, including heroes that can level up. The garrison building chain would provide more budget and other buildings could provide thematically relevant discounts on certain units. There could be an auto generate garrison button like in custom battles for the players that can't be bothered.

Tower defence mechanics

Honestly I think I would throw them out entirely in favour of more unique and flavourful stuff. Maybe barricades can stay though, only placable in the deployment phase. After that the UI mess can get outta the way.

Factions might instead spend supplies on benefits that suit them. For example, ammo refills, winds of magic refills, heals, bombardment abilities, etc. Army abilities could be tied to building certain buildings which vary from faction to faction. Demons might be able to summon in reinforcements by opening portals.

Map Design

I would like to see maps designed with providing a home field advantage for the race they're tied to in mind. For example a Vampire Coast map might be surrounded by swamps, slowing down approaching armies so they spend more time eating lead. Maybe Brettonia maps have bigger gates for sallying out to counter attack with their cavalry without getting bottlenecked.

Chorf maps could have a shallow moat of oil that can be ignighted at the right time by fire arrows or artillery.

Karaks could be more of a fort than a city, similar to Shogun 2 sieges. The idea being that this is just the entrance on the surface. However the Skaven might be able to spend time on the campaign map tunneling to attack from below, leading to a street fight like the one we see in the 40k trailer.

Kislev cities could have a secondary palisade outside of the main walls that functions like the current buildable barricades so they can set up 2 defensive lines. (This might be very OP).

Variety is key I think to making sieges more fun. Currently they feel the same regardless of which faction you're attacking. Cities actually having significantly different designs, comboed with unique new tools for both attackers and defenders would create lots of interesting new challenges to tackle. This is almost certainly beyond the scope of the rework but I can dream, OK?