barrage balloons! sprinkle them around bases, towns, & factories. they wouldn't be too terribly difficult to shoot down I'd imagine, but they may just have the intended effect of reducing/eliminating the number of people who abuse the buff's airframe =P
http://www.fact-index.com/b/ba/barrage_balloon.htmlpros:
Obvious discouragement for large planes dive bombing. Switching to a turret and shooting at them in a dive is easier said than done.
Obvious discouragement for low level bombing.
Would provide defenders a few more seconds to start mounting an attack.
They were moored to wagons with winches, so altitude could be adjustable.
Would keep a furball off of a base & reduce vulching until the enemy cap shoots them down. (maybe that's a con...)
cons:
Would make landing & short takeoffs kind of scary if friendly collisions were on.
Would need friendly collisions on to keep people from dragging enemies straight through the balloons & wires.
I think the pro's outweigh the cons... or I could be completely misguided.