Well you know what I want out of diplomacy, an RPG (animated and voiced at the minimum) where leaders and the various intricacies of diplomacy play out (alien and your own ambassadors, political stances and attitudes you set and procedural gifting system specifically). This should not be something you do every round, but something that you do when you want it and that the AI does procedurally, IE the AI will have a pool of 10 to 20 different approaches and each will play out differently depending on your choices. In those interactions between 2 alien races, ie their ambassadorial staff and you the leader, and your staff and their leaders, will decide how diplomacy plays out, and open up the way to various sub systems, like integrating of aliens into your own society or.. well so on. Possibilities are infinite.
If you wanted to "simplify" the system, think gift , attitude and staff choice you are given, with a multitude of staff options for a diplomatic engagements... there would be no stats, the choice whether 2 races hate each other or not depends on randomized situational decisions you made beforehand, including the first contact scenario, whether you have first contact protocols and whether these protocols end up creating friends or foes...