The current system would be vastly improved by implementing at least some of the changes below:
1. Espionage units should be divided between Intelligence Agents (“Agents”

and Spies. Agents thwart spies at home while spies wreak havoc in foreign lands.
2. Spies and agents should have unique abilities based on their unique roles.
3. More reflective of the combat system, spies and agents should accrue experience points based on their success.
4. Experience points would provide upgrades to spies and agents.
· Possible upgrades to spies: vision upgrades, movement upgrades, upgrades that increase the chances of successful missions, or upgrades that lead to the damage caused by a spy mission to last longer, etc.
· Possible upgrades for agents: visibility, movement, increased chance of detecting spy, increased chance of detaining spy, increased likelihood of obtaining by valuable information through interrogation, increased chance of destroying a spy, better chance of detecting nationality upon detecting a spy, etc.
5. Just as great general production is boosted by success in battle, points earned from successful espionage missions and successful counter-espionage missions would go toward creating great spies.
6. Great spies could be attached to an individual spy unit and gain 20 experience points. If you were lucky, you might hold on to a spy with many upgrades that in the end game has become extremely powerful. Like little ‘Bin-Ladens,’ if you will, capable of swaying global politics.
7. Great spies could also be attached to individual agents and would gain 20 exp. points, making them bona fide intelligence experts so powerful that it would be competent to stop a '911' from taking place within your borders.
8. Just as a player feels proud of his cavalry unit that has been with him since the days of chariots, which is lead by a great general with 10 upgrades, so to would the player come to feel proud of his elite spies and agents.
9. Freshly built spies without any upgrades would be invisible new agents. Subsequent upgrades would alter this. As the system goes, a veteran spy will be pretty damn hard to detect except by veteran agents. Similarly, A green spy is highly unlikely to go undetected by veteran agents.
10. Agents would attack spies just as any other unit attacks. The consequence would depend on chance, the upgrades of each unit, and possible other factors including buildings, etc.
11. One possible consequence of a successful attack from an agent would be that the agent detains the spy, at which point various options would become available to the player. Maybe the player lets the spy go to gain the good graces of your rival, maybe he interrogates to learn something about their rival, maybe he holds the spy hostage for monetary gain, or maybe he just destroys the spy and incurs a diplomatic penalty to try to provoke a war. Choices not passivity!
12. Spies could not attack agents, but they could evade, give false information, or simply disappear. Whether a spy gets detained would depend on chance, the agent’s abilities, the spy’s abilities, buildings, and EP’s for the spy's owner.
13. If an agent did detect a spy, the amount of info about the spy that became available to the agent's owner would again depend on chance, the agent’s abilities, the spy’s abilities, buildings, and EP’s. If the player moved to detain the spy, again those factors would be relevant. Upon a successful detention, the choices available to the player would again take these factors into account. It might be that a player simply has a very rudimentary espionage apparatus and can gain nothing from detaining the spy, in which case, the spy is simply destroyed or returned.
14. Espionage and intelligence would be distinctive. Therefore, it would be possible for a player to give a well developed intelligence apparatus, but not a well developed espionage apparatus, and vice versa. Quick example: one building would provide immediate experience points to new spys, another to new agents. This is similar to barracks v. stables.
15. EP’s would continue to be gained by budget spending and buildings.
16. Espionage and intelligence gathering would be much more closely related to diplomacy and foreign policy.
17. The numbers of spies and agents would be capped. 18 useful spies for 18 civs on the board should be sufficient. Hell, you could use all 18 on one civ if you really wanted. 1 good agent per 3 cities should be sufficient.
18. Emphasis would be on fewer overall spy units, but more options, control, power, and strategy in utilizing the available units.
19. In this system, successful spies stay embedded. None of this returning to capital nonsense. Also, agents could only leave their own borders via ship or open borders. Agents could not detain outside of friendly borders.
20. One of the best changes in this system is that a player would potentially SEE spies in his territory and would be able to track them to gain knowledge of the spy such as its nationality. Having agents running around taking notes on which nations are most active in his land, and adjusting his foreign policy accordingly, would be huge!