The AA guns idea I think would be the best because how strong planes are, I always had this idea but it's kinda big. Kinda of like a research tree where you can get different upgrades for your Nation, because atm 1v1 War is really predictable, normal the Nation with the most cities is going to win unless they got caught with their pants down. So if there was like a research tree it would make war feel unique and be abwl to say your good at war would actually mean something.
TLDR That probably didn't make any sense and I think a skill tree would be cool