The espionage system is one of core feature of 4x game. So i have observe a saboteur unit which can hide from other units and damage district. 

So i think that same concept can be done for a espionage system.

Spy can setup spy post in enemy territory and can do operations. These spy post can make network these network can enhance and support operation or can tackle enemy's intelligence network.

Spy units are important to defend network and can cause spy combat.

What are your  views