How about we have an armor where, if you have more plates, you have more health per plate. That is, with each plate you put on a tank, that side's plates will have more health per plate. So, either the newly added plate will have more health, or every plate will gain health. So, for example if the armor for some reason had 10 HP per plate, the second plate would have 20 HP, and the third plate would have 30 HP, and so on. Or, all the plates would have 30 HP. This would incentivize building fully plated vehicles. Albeit, this will likely require some serious balancing but hey I bet it could be done. Would also make for an interesting armor in say, some new tech tree about building upgrades and resonator weapons.