My younger brother was playing Diplomacy earlier and he had an idea that he wanted me to put up here to get everyone's opinion...the idea is this:
Create some kind of ability or option when dealing with other players (and AI) to help create peace among your own friends. As in, if your good friends (like at least 12.00 to 15.00 with both) (both ways, you like them, they like you), then you could help them with their relationship. i don't know exactly how this would work, but maybe they both just get a 2.00 or 3.00 point boost in their relationships...
This would help you build stronger allies, and allow unofficial "teams" to gain a small advantage.
Edit: a thought? Anyone?