New inventions are a thing in the 3300s... surely. There's lots of tech companies and militaries and stuff all working on different things...
In a world completely created by the devs and story writers at FDev (and their predecessors), the devs and story writers at FDev are in the position of being able to add any new story justification they want for adding a new piece of technology...
In the context of conflict zones though, what exactly are you imagining?
A dome over the whole base?
Cos if you read my edit above, that's not what I'm talking about with regards to that.
Why not just drop a shield grenade if you need somewhere to take cover? or go indoors?
Maybe i'm not understanding your idea properly, but I think it's an answer to a problem that doesn't exist...
At the moment there isn't really any ship > ground combat. I would have ship-based combat zones spawn above ground-based combat zones.
It would add excitement and danger to the GCZs, having ships flying above you, exploding and raining scrap metal on you.
It would also add a new danger to the ship combat - you have gravity and the ground to be aware of, and the possibility of being hit from below.
Stray fire would go both ways.
I would have the AA guns controlable, much in the same way the SRV turret is, for taking down ships from the ground (telepresence from the CMD centre maybe?). I would also have a trigger (attacking faction looses half their troops, or something) which would cause the attacking faction to call in some kind of air-strike - the defending (ship CZ-based) faction would then have a CZ side-quest to take out the 'bombers' before they get a chance to do the bombing run. This would tie the CZs and the GCZs together, allowing them to influence each other, but keeping the results seperate (you can win in the air but lose on the ground, or vice-versa).