I really like the new espionage system. Stealing tech and the intrigue system is awesome. I would say it is the most useful espionage system that has ever been brought into a Civ game. I really like it but i feel it could easily be expanded upon in a number of ways:
1. Causing unhappiness
2. Stealing a lump sum or gold per turn from other nations
3. Destroying nearby improvements. You could station a spy in a city and then use them to destroy the nearby city improvements
4. Destroy buildings
Most of these could easily be used in Civilization 5 and it would really open up more options for your spys. Intrigue is the most useful as well as city-state election rigging but it could be expanded upon a lot in my opinion.
1. Causing unhappiness
2. Stealing a lump sum or gold per turn from other nations
3. Destroying nearby improvements. You could station a spy in a city and then use them to destroy the nearby city improvements
4. Destroy buildings
Most of these could easily be used in Civilization 5 and it would really open up more options for your spys. Intrigue is the most useful as well as city-state election rigging but it could be expanded upon a lot in my opinion.