@Cernel
For WWII scenarios, and thinking about the way the 88s were used in reality (ignoring their use in the late-model panzers), it would be nice if:
AA were treated like any other unit for purchase, movement, etc, except as below:
on defense, AA acts as flak/air defense if any attacking aircraft are present, and as anti-tank/inf artillery if no aircraft are present, at start of defensive fire phase (SBR is just a special type of attack, so AA would be 'flak mode'). No input required from defending player.
on offense, the attacking player has to specify the default mode of the AA guns (flak vs anti-tank) during combat movement, similar to how bombers have to select 'bomb vs attack' in most current maps (I have no idea how complex the coding would be for this, but existing bomber behavior shows that this is possible). If the Attacker wants some of each, move them in two separate moves, just like we do now with bombers attacking and bombing a single territory today.
In 'flak mode', AA hits more rarely (1/6 in most maps), but cherry-picks aircraft only for casualties
In 'anti-tank mode', AA behaves like Artillery in most maps, with better hit chance (say 2/6), but allows full defender-choice on casualties.
If all defending aircraft are eliminated but attack still underway, then AA is in 'anti-tank' thereafter.
So, in effect, AA would just be artillery with a special 'anti-air' mode available.
Cost, cargo size, movement rates, and combat strengths would need to be balanced carefully. If a player wants to buy a bunch of AA rather than 'normal' artillery, then it should be an option, but come with a cost premium. I would think that a slightly higher cost and the fact that they would still count as a placement should prevent unreasonable AA unit spamming.
