The current journal mentions checks to see if the relationship is at either a minimum or maximum value for a treaty to be enacted. I noticed that it lacked a check to increase (or decrease) the value of trade based on relationships between states. Positive relationships should impart a positive modifier to trade, and negative relationships should impart a negative modifier. War would, of course, cut off most trade. Real world analogues abound, so I won't bother listing them.
(tl;dr: +/- relationships should increase/decrease income from international trade)
Another major aspect of diplomacy that tends to tick players off is the implacability of AI enemies, especially in Civ4 and the TW series, and their over-willingness to backstab long term friends. I'd like to suggest that a relationship modifier be included for war-decs that has almost nil impact while low, but virtually prevents war between close friends excepting in the case of heinous slights -- the acknowledged assassination of friendly family or open provocations for war being the only real cases. Meanwhile, the AI should be willing to end conflicts -- even with hated foes -- and actively attempt to improve relations in cases where it would be better served by peace. The most obvious example is in Civ4, in the case of nations that have been greatly reduced by conquest, yet adamantly refuse to make peace because of all the negative modifiers for city conquests.
(tl;dr: make AI backstabs very rare, and AI enemies less implacable)
I would also like to suggest a peace treaty system along the lines of the one used by Paradox in their Europa Universalis games, where conquered territories are de facto, rather than de jure, the possession of the conquerer until confirmed by treaty.
(tl;dr: use peace negotiation and hard limits on territory transfers as a rubber band mechanism)
A I'd like to suggest that a negative modifier be placed on relations if the player deploys significant numbers of soldiers to the border of an AI nation (aka: -5 "You provoke us with your military build up!"), with the AI responding by itself deploying large numbers and generally preparing for war. It's more of an AI based nitpick, but it's obnoxious that the AI rarely responds to something that a player would freak out over.
(tl;dr: have the AI respond to FRIENDLY troop presence on their borders)