Hounds / Tracking falcon / Spy?
My suggestion is to implement some tool which you can track a city dropper. Of course the city dropper has the element of surprise and I think some tool should be implemented to possibly give the defender the opportunity to gain something from being dropped on.
Hounds - be able to send them to the blank square where the city was. They would take a certain amount of time (based on how far the city went) to find it, and send the coordinates to the players inbox. I think it would be fair to cost a certain amount of money ( not coin ) and would cost a significant amount of slaves and they wouldn't be able to regenerate during the time they were out.
Tracking falcon / spy - pretty much the same concept except you'd have to send them while the city was next to you. They would only be effective a certain amount of time ( few hours or let's say 10 min after an incoming attack to work with pinning ? )
Anyways, not very detailed but just a general idea I think could add some fun to the game