As I'm thinking about my mod there is a mechanic I was hoping for that I think the entire game would benefit from.

That is doing things for "favor" or indebting one nation to another (Yes, real life examples abound). Throughout history it has been a common story that 4x games have yet to capture. Nations become indebted to another for a gift of money, support in war, a gift of technical prowess or whatever.

So the mechanic is meant to represent this. As a ruler I might give you something not to improve our relations or in tribute not to be hostile to my civilization but rather because I want you to be indebted to me and mine later.

It could work very similar to how the diplomacy mechanic works right now. What I'm proposing is another diplomatic stat line between each of the countries. If you do something diplomatically as a "favor" you get points in this stat line as a "diplomacy" bonus when you need to ask for something in return another time. Then the favor points go away when they're used to get the debtor to do what you ask. To avoid abuse, going hostile to someone whom you owe favor would cause a diplomatic penalty with all nations equal to the amount of favor you owe them.

(This would have been really nice in Galciv when I was giving stuff to allies all the time to beaf them up and then having them reject my requests later for things I wanted them to do.)