Oh ho ho, i've been waiting for this. *Cracks knuckles*
Not going to go into huge detail, i'll just rattle off the general ideas
I'd love for side missions to have more of an impact. i.e. "Stop a convoy from reaching their destination or have to deal with an army of tanks at the next AO"
The potential for enemy reinforcements to come into the AO would make it quite a bit more diverse.
If players fail several defenses in a row, they have to defend the airfield from an attack
Specific squad units for people wishing to drive tanks, with similar rules to the pilot slots.
Several HQ like structures in each AO, with only one of them actually being the real HQ, but all of them having enemies.
Enemy bases spread out across the map, with regular patrols and support units.
I really should put this next one in FOB feedback but i'll put it here because i'm lazy, some of the FOB's are extremely far from the AO they are assigned to (Sometimes 7+km away)