Since Total War: Warhammer 3 adds the China-themed army of Grand Cathay, for whom "making great big walls" is kind of a thing, it's fitting this is the game in the series to rework siege battles. As Creative Assembly explains in the above video and a blog post (opens in new tab), Total War: Warhammer's least popular battle type is being significantly reworked.
A wider variety of siege maps, themed more tightly to the various factions, should shake things up. "Cathay and Dwarfs build highly defensive fortresses set into the sides of mountains," for instance, "where their firepower can be exploited to its fullest." Besieging Kislev will mean fighting through tight streets, while the skaven and greenskins have "more ramshackle fortresses that are no less deadly", and so on.
Some cities will get more than one wall, with Cathay's Wei-Jin mentioned as a fortress protected by three layers of them. It seems like infantry will still be able to scale walls with ladders they apparently keep in their pockets, based on the video which shows daemons scurrying up one. Certain attacking units will be able to damage walls directly, like they already do with gates, if you're more into knocking them down. Defensive towers on walls are getting bigger capture areas—the zones that defenders need to hold to prevent towers being taken over—meaning defenders can spread out further.
Past the walls, defenders will have buildable structures like those seen in Total War: Warhammer 3's survival battles. Holding key locations gives defenders supply points to spend on barricades, towers, and traps around those locations. They can be dismantled and rebuilt if the fight switches to a different area, though it'll take time, and if a point is taken all the structures around it are lost.
Fighting through the streets will be made more interesting too, with settlements now multi-leveled. Bridges and overlooks will give defenders positions to be docked, as they are on walls.
Similar tweaks are being made to minor settlement battles, with more maps and the same system of capture locations and buildable defenses, though minor settlements remain unwalled. They're being made bigger though, presumably so you can choose your own angle of attack.
All this won't count for much if the AI isn't able to effectively use the new tools, or if it's not aggressive enough to lay siege and you never get to use the defender's toys. Creative Assembly says it has "improved the AI, not only to handle all these new elements, but also to provide more of a challenge whether you're attacking or defending. Flyers and fast-moving units will now prioritise useful actions like preventing sallied-out cavalry from attacking ranged or siege units. The AI also has a greater understanding of how different areas of the map are under threat, and when they should withdraw to prevent losses."
Playing a Total War: Warhammer campaign means besieging a lot of settlements, and it can be tempting to autoresolve them once the novelty wears off. We'll have to wait until next year to see how effective these alterations are at shaking up siege battles, since Total War: Warhammer 3 has been delayed into 2022.