Since AA guns (with the exception of ships) are fixed to specific locations, and in general, those locations are dumb as hell... I'd *LOVE* to see AA vehicles added to the game. Give the allies the standard quad-.50 browning truck mounted guns, and give the axis truck-mounted flak-38... the hitting power should be pretty close (4 MG bullets versus 1 AA shell), and it would be historically accurate. (it'd also be able to fire at a zero-degree elevation) The axis would have anti-tank ability, and the allies would have better anti-infantry... so it wouldn't be balanced in that sense, but hey, war isn't balanced.
You could have 2 of these driving around to guard areas, and they could actually do a good job against ground targets if used in an assault/defense role. Basically take an APC, and don't let anyone ride in it, just put the gun/guns on the back, and there ya go. Just a couple of those shouldn't make flying a suicide mission, but it'd actually make pilots have to worry a little. (and it'd give dedicated AA guys like me something actually useful)
I know 4 MG bullets would probably have more hitting power, but if you make the guns overheat like the other MGs, that'd lower the "average" firepower to probably keep it pretty close between the two options.
Good? Bad? "BBF, what the &@^#$ are you thinking"?