I was playing World of Tanks a while ago and I was thinking of how a MUCH more simplified version of it's armour system could fix the balance between AA guns and AT guns.

Armour would work in 5 dimensions + 4 turret sections (for tanks):

frontArmour
backArmour
sideArmour
topArmour
bottomArmour
frontTurret
sideTurret
backTurret
topTurret

Vehicle armour piercing would be an effect on the ammo as well as the gun (the ammo can add penetration to the gun's base value) so AT rounds on tanks could do more against tanks while HE would wreck smaller, less armoured vehicles. The armour penetration system would work as such:

My M4 Sherman shoots a Panzer 4 in the side of the hull. The gun on the M4 Sherman has 92mm of penetration, and the Panzer 4 has 30mm of armour in the side. POW! 62mm of penetration so the damage is 100%! The brave Panzer 4 turns it's turret around, and fires a shot as the front of the Sherman's turret. The Panzer 4's gun has 103mm penetration while the front of the Sherman's turret has 73mm. 30mm of penetration, so the damage is reduced somewhat but it still does a fair bit of damage. The battle goes on, but that's another story.

This could balance AA guns a lot due to the fact AA guns were not designed to penetrate armour, so you could give them a weak armour penetration value so that any heroic soldier that tries to save their squad by turning their AA gun against a T-34 would get a nasty suprise, while that same soldier doing the same against a Messerschmidt (if the user could make a hit) would be rewarded by not having an enemy plane in their area.