Yes, here we go again, another Demian idea. I love in Galactic Civ how you can bring various items to the table in diplomatic negotiations. Planets for money, money for peace, planets and money to declare war on an opponent, etc. But one thing that I've always thought was missing was "diplmatic capital." You know what I mean--- when you do someone a favor and they "owe you one."
Diplomatic capital can be a currency that represents the abstract total sum of favors that you have done them and it increases every time they get the good end of a deal or you do them a favor (everytime they do YOU a favor, they gain diplomatic currency against YOU). As your currency with a sovereign builds, it gives you a passive bonus to your total relationship with that sovereign which will marginally improve the probability that they will accept diplomatic demands/offers.
Now, to pre-empt all of the "but this feature already exists in other games" that will surely ensue, I know that most other stretegy games have the above mentioned element, Galactic Civ and Civ 4 included. The difference, though, would be that you could spend that diplomatic capital like currency to improve your odds of gaining a treaty or succeeding in your demands. For instance, you might have a decent relationship with a given sovereign, but a request that he declare war on one of your foes may be unlikely to succeed with out a bit of "umf!" behind it. So to improve the probability of the demand succeeding, you could toss some of your diplomatic capital into the heap to turn the tables. From a role playing perspective, you would basically be approaching the sovereign and saying, "Hey Joe, remember that time I killed that tribe of hobgoblins for you? And that time I sent your city of Acacia that extra food to get them through the winter? Yeah, well, if I recall correctly, you said that you owed me. I'm calling in that favor."
If they decline the demand, they must match some of their own capital against yours or suffer some kind of temporary penalty that scales with the quantity of capital spent (this feature is necessary because the AI will be gaining capital toward you as well). This penalty could be internal instability, lower research rates or lower commerce rates to represent your citizens' discontent at your dishonor, dastardlyness, etc. But political currency could be used for more than simply making treaties or demands. It might also be used to deflect unjust wars waged against you. If your diplomatic capital is high against an opponent that decides to declare war against you, you can use that capital to shame them into retracting their declaration. What's more, if a collection of opponents attempt to gather in a campaign against you, you might pin point specific members that you have built capital with and pressure them with your capital to drop out of the league ("Mitch, Jeb, Frank, I can't believe that you would do this! After all that I have done for you 3!")
Anywho, let me know what you think.