With metal bases I think should come stronger doors. My suggestion is that there is a new door called Reinforced Metal Door which would only go on Metal Doorways.
The Material requirements could be x10 Low Quality Metal. It would take four explosive charges to take it down.
No one goes through walls on Metal bases and they require a crap ton of Low Quality metal. I think strengthening doors will enable users to push more towards end game bases which are metal.
At the current moment everyone just goes through Metal Doors rather than the metal walls.
We need more risk factor in raiding a metal base than just going through all the metal doors cause its cheaper that way. Make the raiders take the risk of picking wrong doors. I think this will add more risk and a bigger reward for those raiders who go all the way through the doors or walls.