My idea is simple: If a ship is flying towards your city, you could send one of your own ships(or several) to intercept it before it reaches the city. You would also be able to do this if the AI are attacking a different city to the one your sending ships from(however the airships you're sending would have to be fast enough to reach it)
Very good point, and should be totally possible. I have added it to the todo list. :)
Oooh, there could be a new ocean background that would display when intercepting ships over the seas. Grounded ships without enough suspendium to fly could float in the water for awhile, but gradually sink.