It should be a turret that faces one way at least. A gun that can spin around is just boring. Adding it so they have to turn and face the enemy will make the dogfights more interesting and longer. Adding homing missiles will make the gameplay pretty bland too, but if you give them like 1 or 2 and they do like 50% damage each then it should make it interesting.
Making AA guns that are AI controlled wouldn’t be to much fun, because it would add to the overall gameplay if they had to defend their base AND attack the enemy base. If you were to add, oh say AI controlled tanks that would go to the enemy base and would need to be bombed it would add to the fun.
As for the planes it would be interesting to have a few different types like Bombers, Dogfighters, Escort, etc… Destroyable bases would also be a plus, which could include things like 20 hangars to a base, each destructible and the objective would be to destroy the hangars and AA guns. There could also be like generators that would lower spawn time if destroyed, or ammo depots which would give more ammo on spawn, so they would want to take this out.
These are just suggestions, you don’t need to take them into consideration or anything. Also if you are looking for any help in mapping then you can PM me.