Personally, I'd like to see something that relies on the time/resources that were spent building a wall. For example, when building a brick wall, you would be able to pick from 3 options that each require a different amount of resources. The first option would be the weakest requiring the same resources brick walls do now. Those weaker brick walls could be upgraded to stronger ones later on. However, I'm not sure if it would be better if the appearance of the brick wall would change based on how strong it is. If they all looked the same only the people that built them would know and could get away with having weak walls.
I don't think that the soak or the strength required to bash brick walls should be changed though, the upgraded versions should simply take more or a lot more time to break with the same stats. If stronger walls are implemented however, I would like to see new tools to break those walls be made available. Battering rams that require a few people to operate for example.