You can find something similar to this now in Demographics. The military advisor will also let you know if civilizations are stronger or weaker than you in various degrees. It's not exact numbers, types of units, or what specific techs they are ahead. But you can definitely tell by Literacy rate if you are ahead or behind, and who is the leader and who is in last place. Same for production, wealth, military, etc.
I know that I use info addict every game. Of course, it gives you only general info, useful yes, but general.
I would rather have the spy determine more exact info (The number of tanks, planes, locations of units etc)., but as a process over time, not all at once. Of course, the spy's experience should be taken into consideration for this with some sort of bonus. The bonus should not be too powerful.
Let's say he started a spy network in Cologne, Germany and he begins gathering military info about the city. He learns after 10 or so turns the city has 3 tanks in the city radius and an AA gun within the city. However, because of his experience and the longer arm of his trusted contacts, he learns that Berlin has a massive military force built up. Now at first the spy gets this extra info, but it still is not exact military info, like what this massive force consists of, and what it will be used for. This will take further turns for the military picture to develop. Meanwhile the spy is trying desperately to avoid the gestapo. Who are way upset that he practices sidism. As time goes on the spy begins to figure out that Germany is mobilizing an army to invade Russia, with some 5 panzers, 10 infantry, 2 paratroopers, 6 artys, 5 fighters, and some bombers in support. Exact info in my book is more interesting, than just 'they are going to backstab Russia'. Uncovering something like that would be equivalent to cracking the enigma code.
The spy is from France who happens to have an embassy with Russia and fairly good relations. Most likely France will tell Russia of the impending invasion by Germany. At the same time France hopes that Germany will march headlong into a surprise Russian bear trap. Since this would give France an excellent opportunity to invade a weakened Germany, so French cities, captured earlier by the Germans, can be liberated.
One thing that could help is if they gave the ability for your spies to work together with another civs spies, based on certain conditions. One may be that the two civs need to have an embassy with the other. This could result in a risky business. Which could lead to either spy turning on the other. Or, they could work together pooling their resources together to get a better picture of what they are trying to figure out.
It just seems to me that religion has tons more options than espionage has at this point. Well as far as we know. The couple things spies can do seems rather dull. Maybe dull is too harsh a word, but you get the point.
This should certainly be a topic for further discussion. How to make espionage more immersive?