I see espionage ideas as most promising for small countries with lots of diplomatic relations. Have you been able to alter the course of world history from behind the curtains? If so, how?
The gem in the Espionage idea group is Infiltrate Administration. If you play opportunistically as a smaller power, your ability to see into the enemy movements can give you the window of opportunity to strike at the best time.
In a Venice game, I use it to tell me the best moment to DoW on Ottos.