Two features I would like to see in Diplomacy are improved resource sharing and "submission".
Improved resource sharing: The ability to share objects (ships, structures) with an ally or teammate
Submission: The act of obliterating or threatening to obliterate a besieged enemy, giving them the option to "submit" and become an "unwilling ally" or to be obliterated. This, in effect, would create a cease fire, regardless of the number of diplomatic points. In conjunction with improved resource sharing, the player could then choose to "build up" the ally, improving relations, and eventually having a new ally to assist with taking over the universe.
Historical reference: World War II - Japan Vs. USA
It's about thinking longer term. I could destroy one of the other races early in the game, or I could pummel him, offer a cease fire if he joins my "empire", and then conquer the galaxy with him as an ally.
I think this could add a new dynamic to gameplay, and would definitely fit the "Diplomacy" theme while still being true to life.
What would be required to make the changes... I don't know, since I don't have access to the source 
but my guess is:
Improved resource sharing: could possibly be accomplished by adding a mechanism to change the owner of objects. I'm presuming that the objects in game have an "owner" property. This could be accomplished easily by adding a mechanism to modify the "owner" property.
Submission: This would be slightly more difficult to add to the AI behavior. At what point does an AI become "willing" to submit? How much pummelling does it take? Will they be allowed to request a cease fire if getting pummelled, or does it need to be something that the pumeller offers to the pumellee? Off the top of my head, they would become open to submission if the following conditions are met: 1) They obviously cannot stop you from running over them. You have destroyed their fleet and you have a massive fleet, so any ships they DO build will be destroyed immediately 2) You have dealt so much damage against them vs. how much damage they have dealt to you (say, 10:1 damage taken to damage given ratio) 3) Your fleet surrounds their homeworld. There are others - I could list some of the conditions for submission for days - it is really an open ended idea with many possibilities.
