How bout just randomizing the positioning of the Ack guns on each field? It gets too easy to take bases/etc when everyone has memorized their positions already.
If you have a decent video card picking out ack guns isn't that tough. Moving them around on a field would make things a bit more interesting, but no harder.
To give you an idea of how bad a horde is. We use to de-ack a field with 3 guys in one pass each. What you do is time your runs together which spreads the ack fire out and reduces the chance to get hit by it. Now, if the horde had any skill or leadership they could have each guy hit one ack gun all at the same time and not lose a single plane to ack and have the field or town de-acked in one pass, or in seconds. Thankfully they don't seem to have the skill or leadership to do this.
Nobody likes to defend. If something was added to the game that made it profitable to defend more players would do it. As it is now, it's a boring, time consuming thing, or it is full of frustration as you die over and over again with very little success to show for it.