Most people know what diplomacy is due to so many games being diplomacy-based. Basically it would be a Diplomacy game mode where everyone would start out neutral with eachother. Then each person would have a "Negotiate" button next to their name, which would bring up a negotiation tab. There would be options for an Alliance, Form Union (Basically a big alliance that can have more than two nations), Declare war, and Make Peace. The other player would be notified of war being declared and would receive the offer to accept Peace and Alliances/Union formations. I think a lot of people would be happy to get something like this,
I’m pleased to announce that peace treaties / diplomacy games can now be played using the new mod framework!
Check out the mod “Advanced Diplo v2”, which has a war/peace/allies system, and a lot of other neat features that work with this!
I'd bee interested in this if it allowed you to transfer armies to your "teammates" after forming an alliance.