First I would like to say that I am very happy to hear about Civ V being released this year.
I have been playing Civ 4 for the past two years on a regular basis.

In order to make Civ 5 more interesting I would like to suggest adding a new “Double Agent” spy unit.

Initially all Double Agents will start as normal spies.
Under certain conditions, buildings, techs, and promotions some spies could be converted to double agents.

Double agents can then work against nations without negative consequences to the player who deploys them.
In addition to that they can stir some trouble between two other nations to a point that they can cause them to go to war against each other.

Here is how it can work.
The human player at some point is able to promote a Spy to a Double Agent.
At this point I am not going to get into what conditions and promotion should allow for the creation of a double agent.

The human player then decides what nation the double agent is pretending to work for.
Let us call it nation A.
So at this point this double agent is working for the human player but the other nations think he works for nation A.
When we use the double agent against nation B, nation B thinks that the double agent belongs to nation A and not to the human player.
This will create negative points towards nation A from nation B point of view (if the double agent is discovered).

As a result we can use double agents to stir trouble without worry about diplomatic consequences towards us.
At the same time we can use several double agents consistently against nation B causing them to go to war against nation A.

I think that such features can be very fun to have in Civ 5.

Of course, the condition that allow for a spy to become a Double Agent should be strict enough to insure that Double Agents are hard to come by.