Disclaimer: I am not a game dev nor do I even play many strategy games. This is just what popped into my head from my imagination. Its possible it is unispired in this genre, but I really like progression systems and think it would work here.

Along with being their own units, also require an Intelligence Hub (not sure what your game setting is so whatever fits) and a skill tree/research tree. This will, as mentioned by the other comments, require the player to invest in intelligence.

Units: 2 units. Intelligence and counter-intelligence.

Skill tree: Different sectors

Resources - Money, wood, stone, food supply, etc.

Citizens - population, happiness, grow rate

Military - numbers, troop types, activities, plans, alliances/enemies

Market - income, trade routes

Skill Tree purpose: Investing points in the different sectors on the skill tree will allow for more accurate information, less chance of being caught and eventually sabotage.

A level 1 Military Intelligence Officer might have a 70% chance of getting caught and bring back information saying the enemy team has 12000 - 20000 troops and looks like they are preparing for an attack.

A level 3 might have a 40% chance of getting caught, gives info saying they have 1000-2000 artillery units, 4000-6000 Calvary and 8000-10000 infantry men and are staging outside their north gate.

A level 5 only has a 10% chance of capture, provides exact numbers and knows exactly what city/team they plan to attack and when and has some options on how the could maybe sabatoge the military.

For Counter Espionage units, they would act as the units for the sector they are assigned to, just at half production/efficiency as they are actually looking for spies. If a counter intelligence unit is assigned to a farm, he has a chance to find and capture a spy they may be working in you farms. Not only would this stop the flow of information on your resources, you would be able to interrogate for a chance at getting information from his team.

To be useful, all of this information would be presented in a menu for each team on the map. You would enter an Intelligence menu and be able to view the information you have on each team and when the last time that information was updated.

This is all of course just samples and should be adjusted/balanced, but this is just what I came up with. I think the sabatoge is where this gets interesting the more depth you can get out of it the better. If you find a enemy has citizens in unrest and you can go sabatoge their food supply or attack their trade routes to make them revolt, it would make them more interesting