I suspect this is yet to come in a DD but I still wan't to know more about this.
Diplomacy is usually disappointing in grand strategy games.
Pax Britannica really worked very well in this department. When you had those "conferences" (like the real ones they had in Berlin etc), you really feel that you are discussing important matters with lots of room for backstabbing and power brokering. Colonies divided, borders were drawn with rulers etc.
I suspect this won't be as exciting in solo but for PBEM/Multi it would be nice to have a nice interface that emulated these large conferences.
Advanced Civilization(AH) had this nice trading phase which added a very nice game element. You had the opportunity to trade away revolts and other nasty events.
Something like this should be explored for PON, or at least a future game.