Problem: Diplomacy doesn't feel fun. Diplomacy sounds as if it should require intelligence, intrigue, beguilement, etc; but it doesn't.  What it real boils down too is very simple abuse of the AI as it tries to fake interest in the world around itself. I enjoy it about as much as Frogboy enjoys tactical battles. 

Suggestion: Make is so you have diplomatic currency that can be spent as an either or scenario. I spend 10 diplomatic points and you either sign a trade treaty with me or your city gets 10% unrest for 10 turns.   I spend 30 diplomatic points and you either sign a trade treaty with me or your city gets 10% unrest for 30 turns. I spend 100 diplomatic points and you either sign a trade treaty with me or your city gets 10% unrest for the rest of the game. 

The rewards and consequences could scale and vary, but this would basically open up the AI actually having power to get you to sign treaties that are not necessarily in your best interest all because they have spent the time to become diplomatic instead of war focused. 

EWOM tried diplomatic currency, but it was completely a cheese tactic as I could care less how much currency the AI used in a deal with me, but they cared a lot if I put up a lot of points. My suggestion would make it so I actually cared because it is going to actually effect me.