Meh, when I bomb something with an airship, all it causes is 12% damage. No collateral there.
Why can't airships cause collateral damage?
A flak gun would be nice though and it should not go obsolete (make it have an advantage against tanks because an 88mm flak gun can be an anti tank gun) because we still use flak guns today
First thing, people are complaining that the airship does even that much damage. Historically, they weren't very effective weapons. Collateral damage would make them more powerful than that, and they may be too powerful as it is.
An AA Gun unit would make sense, as they were used but not of great effectiveness. Get them with Artillery, that would make sense. Modern AA guns are more effective, but flak guns rarely killed aircraft. The flimsy ones in WWI, yes, but not often.
A big problem is with when they become available. Physics really should get observation balloons -- shorter ranged and only recon mission. Combustion is necessary for Zeppelin airships, because what else makes them steerable?
Why are they in the game? I'd guess because the AI could be programmed to use them, and they were of historical importance. In our real world history, true dirigible airships flew barely before airplanes -- the first Zeppelin in 1901. They actually developed in parallel, competing flight systems.
Observation balloons, those were available much earlier, and were important. A better recon range definitely wouldn't be a bad thing. But the AI might spam them and use them foolishly, which wouldn't be good for the game.
A lot of units could be added. I think I'll do a thread on that. But unless they are fun and useful in the game, they aren't worth doing. Marathon speed players would get much more use of units which obsolete in a tech or two, while normal speed and faster wouldn't be so likely to use them. The modern era definitely lacks some developments. WWI flight and submarines are barely represented, and the cold war 50s-60s likewise. The revolutionary developments in military technology are glossed over.
One more problem with airships is that they obsolete with flight. In the real world, airships remained in service through WWII. Their military usefulness essentially went away when better recon aircraft came around, but they were still useful for recon and were good submarine hunters. I figure they shouldn't obsolete until a unit to replace them gets built, and that should be advanced flight or rocketry (whichever one we want to allow the submarine recon promo for bombers).