I like Airships—it takes me back a bit. However, I think it could do with a campaign mode that got a bit more depth without adding too much complexity.
Right now a city has... Itself. That's it. It can defend itself with static defenses and its fleet, but once the enemy barrels through those then it's game over for that city: it's just been conquered.
I propose making the initial city defenses strong enough to make a head-on assault inadvisable, and also give each city three or four points outside that it controls. On the simplest level these could just give you cash. They'd be worth fighting over, and they'd be worth defending.
If, after that was implemented, a bit of complexity was going to be added to the campaign to get a bit more depth, you could implement stuff like strategic resources—you need to acquire enough suspendium to build your huge fleet, and enough coal to power it. Also toss in coal costs for strategic movement, so coaling stations are an added value. Maybe even a rudimentary trade system, so you could have an excuse to add in pirates for the players to watch out for.
And if you really wanted to show off with complexity, you could make conquering a city not automatic: destroying the defenses would merely allow you to send in ground troops. Or, if that would take too many troops away from you, you could make them give you their stuff.
I like those suggestions. :D Having more detailed cities and more points on the map to interact with, including smaller ones, is definitely a thing that needs to happen. So a deeper campaign mode is in the plan, though not for a little while yet. For now, any suggestions are highly welcome, as I really haven't settled on a precise feature set, more of a list of things that would be cool. :)