Keeping it short.
I think something that could make the game feel more unique from previous titles and focus more on city management - something basically all TW games seem to lack these days - is changing up the way we form defenses and garrisons in cities.
So if you want walls and extra troops in a city (minus ze Lady's people and my guys up in da norf) you play the level-up-this-city game, hit tier 2, get the garrison boost, and then play the level up game again and finally you have walls and a city with only 3 more building options left.
I feel like this really is a bad way to go about that. Offense/Defense is the name of the game in TW, buildings and managing them are mostly restricted to unlocking units, buffing towns, etc. So instead of "forcing" the walled settlement that renders most minor towns into 3 building slots, let's make defending cities something you can actively manage - or ignore - based on your needs.
I'm suggesting that the original garrison building be removed, as well as any units provided, and instead add a new UI which has the sole purpose of allowing you to choose what you'd like to recruit to "garrison" or build defenses for the city/village.
Let's say you are the Empire and you want to manage Altdorf's defenses. You pop up the Garrison UI. You can choose which units or heroes to recruit, build up your defenses, add towers, add cannons to the ramparts, even add additional layers to the cities defense (think Castles in Medieval II). You choose what the Garrison army consists of, how the defenses are prepared, and if you like you can even transfer units from your Army to serve in the Garrison. The Garrison force will level up from battles, training (which you'd pay for), etc and you could also choose to pull out any unit and add it to your own army.
Each unit recruited and every defense option chosen has a monetary cost which is deducted from the provinces tax and possibly other resources. Another point is cities or villages that are far from the front will likely be less manned than those that share borders with other factions, which in my opinion would make for some epic Skaven/Beastmen ambushes catching you unaware.
As a side note, I think this could also be a really great way to incorporate Resources into actual use and not just something you build for more Trade Money. For instance, you own the lumber resource - excellent! - whatever defense that requires Lumber is now cheaper since you won't have to import it.
This is WARHAMMER! Choosing your troops is key - so why is nearly every single 1 of my settlements defended by the same clone army? Let's make it so when you take losses to a Garrison that you actually feel the sting from losing a unit you've been training for X amount of turns, or that satisfaction when your custom Garrison army successfully defends against a much superior force.
Remove all garrison stuff, no more forced garrison building for walls/defense. New UI "button" to recruit your own custom garrison, build defenses, add artillery/firepower, even expand the map based on your choices. Garrison units level up just like any normal unit and can even be plucked and placed in your own army. No more clone garrisons. No more 3 building slots cuz muh walls.