I know this would take a while and be far down on your todo list Noblemaster, but I think there are some cool things you could do with diplomacy.
I think their should be the ability to make treaties (mainly for multiplayer). You could include lots of things in these treaties. There could be payments (One time or continuous). You could set mutual boundaries: for example if you are both invading a nation you can agree who will get what, like the allies after WW2, and all the land you would normally take goes to whoever you formed the treaty with next turn. You could also include the other types of diplomatic relations in the treaty, and sign these treaties with multiple nations.
Very advanced, but I like it