1. Fortification are not as useful as you think. If enemy has found your HQ and you unfortunately encountered a russian specop squad on the ground that you didn't kill fast enough, their squad leader would call in SU-25 to napalm the area. It is instantly lethal within an area and negates fortification.

2. Most outpost you capture will change hands multiple times. those lovely looking fortification never survive long and their apparently usefulness in defence is overrated. Enemy attacks always possess air power which render most barriers meaningless.

3. You can build bunkers in the existing game, perhaps not enough to your liking. If you didn't know, press y and look for the building section. Bunkers are saved but not barriers and trenches. There are hard limitation to the save system.

4. Steel hedgehog and dragon teeth? This is fundamentally an insurgency scenario. Building a Mannerheim line is counter to the theme. Those lovely AI you post are purely there to buy you a bit of time to respond (should you decide to do so) and will never stand up to a full scale attack. Play for a while and you can start anticipating the direction of enemy ground reinforcement. We often preposition ambushes with IED along reinforcement roads before attacking an outpost. Be creative.

5. Static HQ is not encouraged especially toward end game. Perhaps at the start you may have a base at the same spot while operating in a single area to gain traction. But that HQ needs to be well hidden. By late game, your pace of attack need to be very fast, even bypassing enemy outposts, to achieve the objective of capturing all airports and >50% of populace. That means you will need to move HQ along your frontlines or you risk needing to teleport 1-2 min to fast travel and not being able to respond to emergencies. Reason is that at war level 10, if you haven't drained the enemy faction of manpower, their pace of counterattack is blistering and they may outpace your speed of conquest.

6. Your HQ should not be routinely found by the enemy. Avoid posting it near cities or any outpost/resource/factory/airport that you control. Avoid locating under the flight path of enemy air reinforcements. Some cities have scout teams that patrol a very large radius and could easily detect your HQ if it is too close. At lower war level, losing a territory carry a very low risk of HQ detection. As your war level rises, each time you lose territory, there is a significant increased chance of generating a defend HQ mission. Russians more than the americans tend to firebomb if you allow their specops to engage you on the ground. Moving HQ 1km away and avoiding the fight is a perfectly valid way of handling the defense mission.

Oh sorry if you ignore my rant above and i answer your actual question. It doesnt seem like a complex build system can be implemented as the save system cannot preserve it...that mean you will have to rebuild it every time you start the game. In the last meeting, the community developers are not considering this specific area. If you really crave bunkers, turn on your zeus and spawn em in. On the official server, the admins sometimes zeus in a nice fortification to make things look neat. Functionally useless if an HQ attack is underway.

p.s. I am referencing alot of RHS mod faction rather than the vanilla Russian=CSAT, US=NATO. but fundamentally it is the same back end script.

"Fortification are not as useful as you think ..." <- This is not always correct due to many reasons. One steel hedgehog placed in a right place in a right time can slow an invasion of heavy units. If the game engine is not capable of using AI, this does not mean that in real life such things are not used. Etc.