I do agree that construction or rather reinforcing bases would upgrade game play but you usually see this on bigger maps. Although BF6 has big maps I'm not sure they are big enough to support what you are going for.
With that said what they could add aside from support walls would be for current game bases to not have sand bags or fortifications and instead the player would upgrade it to do so (this would be specific to bases).
They could probably add extra functionality to upgraded bases buy adding supplies creates and allowing players to change class
