Heres something else Id like to see in Civ IV Assassins. Everyone knows of the Japanese Ninja, but many cultures, including the Ottoman and Persians, used assassins to attain political goals. You could run them much as you would operate a Privateer. Send them after a Leader. Align them with the Espionage function a spy who could give them the knowledge of the existence and location of a Leader. Give them an attack of 6 and a movement of 3, but only a defense of 1. If they are discovered, they are easy to kill, but give them the capacity to attack from two tiles away. If they are able to launch an attack against the city where you have your Leader, they only have to overcome one defender (your best) and then, if they overcome him, they kill the leader. If their connection to your culture is uncovered by an opposing spy, your reputation suffers a devastating blow.
This gave me my next idea: The top rank of Barbarian, above Raging, should be Terrorist. They would come into being somewhere equivalent to post-industrialism. A function of your Espionage units would be finding, uncovering and destroying terrorist cells. Like barbarians, they would operate under their own agendas, but you could use them. Say your spy uncovered a terrorist cell in one of your opponents cities. He would have the choice to share the intel with the host culture (earning major rep points), simply destroy it (also getting rep points), or infiltrate it and turn it to his own uses, much like a Privateer. The cells activities would then be turned against your opponent. Of course, if your connection with the cell was brought to light, your reputation would never recover. Uncovered Terrorist cells would be left to their own agendas, occasionally blowing up one of your improvements or structures.
This gave me my next idea: The top rank of Barbarian, above Raging, should be Terrorist. They would come into being somewhere equivalent to post-industrialism. A function of your Espionage units would be finding, uncovering and destroying terrorist cells. Like barbarians, they would operate under their own agendas, but you could use them. Say your spy uncovered a terrorist cell in one of your opponents cities. He would have the choice to share the intel with the host culture (earning major rep points), simply destroy it (also getting rep points), or infiltrate it and turn it to his own uses, much like a Privateer. The cells activities would then be turned against your opponent. Of course, if your connection with the cell was brought to light, your reputation would never recover. Uncovered Terrorist cells would be left to their own agendas, occasionally blowing up one of your improvements or structures.