So, I was sitting here browsing the forum, and an idea just popped right into my head.
The game already has static ground defenses in the form of bunkers; I figure static air defenses wouldn't be too far-fetched. I'm thinking something along the lines of hot air (well, probably suspendium in this case) balloons that can shift up and down after being deployed, but have no ability to steer. They could range from simple rifle modules to aerial mines that need to be taken out; more of a nuisance enemy than anything else, expending valuable ammunition clearing them out so you can maneuver.
Note: This is actually partially inspired by Zarkonnen's description of the tech level (late 1800s/very early 1900s); so attached is a picture of a pre-WWI era observation balloon.
Update: Blame insomnia, but I found the sprite sheet and kit bashed an example of what I'm thinking of. It's sized up to show detail, but each section is a small ship module. I may just have to move this topic to the modding section if I keep it up.
Edit Edit Edit Edit Edit: Finally got the link to work. Completely not used to markdown syntax.