Random idea that just popped in to my mind:

What if city battles were completely removed, in a way that when you fight in a city, the "layout" of the city would actually be just a field where the two armies meet.

I am under the impression that the layout of cities is quite modifiable, just that walls and buildings are really hard to change. What I'm suggesting is that all the buildings and streets and walls would be took of and leave the area with just fields and trees.

Pro's: End of Siege:Total War and more field battles, no bad pathfinding in "cities", eliminates the need for modified buildings.

Con's: No more siege battles. In other sense would also spell doom for sieges, as there is no reason to wait.