Correct me if I am wrong but from the world view you can only spy one City at a time.
If that is the case I would like to propose a new type of spy and this spy does not go to any destination.
It is activated from your City in the 'World Map' and of course to activate it one cannot be in protection mode or vacation mode.
By clicking on a new icon from your City on the world map you can see all troop movements upto a given radius between different cities.
The radius this spy can 'see' is a function of one's watchtower level.
The higher the watchtower level the further this spy will be able to 'see'.
The starting watchtower level for this spy to be able to be activated will have to be higher than the highest current watchtowetr level by at least say 2 levels. Hopefully the resource cost for getting to this watchtower level wont be easy to obtain.
You only et one of these spies per City.
The cost of the spy should not be that cheap either.
Also once this spy is activated and it does a spying 'mission' there has to be a cooldown period before it can be activated again.
I propose one mission every hour.
With each increase in watchtower level after this spy is able to be activated the cooldown period gets lower by a small margin.
I would think the spy reports are seen in a similar format to what one gets when one spies on another city currently but all one sees is the troops who are in transit not the troops who are in any city.
The report will show what the complement of troops are and how many of each troop is on the move and also how long the troops are from reaching their destination.
One can then use this information to see if one can attack a City that the troops are returning to.