[AdmiralZhao]

There is a board game called _Dune_ which has an interesting alliance mechanic. Basically, nations are only allowed to make alliances during times of Confluence, i.e. every 10 turns or so. After a Confluence, alliances are unbreakable until the next Confluence, though the exact degree of support given to your allies is of course up to you. You are considered at war with everyone not in your alliance, and an alliance maxes out at 3 members. I think this might work for the big game. You could have all diplomacy be public during the Confluence turn, and then after that you are only allowed to PM your allies, if any.