Way back AA guns were able to be used against infantry. Frankly I think this should return since now every faction has a single barrel low magazine 20mm gun and more firepower is good for both sides. Historically AA guns were found to be effective multi-use weapons against the ground. But if AA will remain neutered then I have a better idea. In a way this at least keeps the role of the AA gun distinct while adding a new tool to our arsenal
Autocannons can be added as a unique Engineer building for the Assault squad. I say Assault because frankly autocannons don’t really match any other squad, and I think autocannons are too specialized to be a basic Engineer building. Autocannons are rapidly firing fully automatic guns firing large caliber shells. They have a greater range and damage than machine guns. Autocannons were typically used on planes but some were pressed into ground use in this desperate war. Autocannons would do more damage than machine gun nests plus splash damage but their high rate of fire would make them overheat easily.
USSR: ShVak Cannon
Germany: MG 151
Japan: Type 99 Cannon