Hi Chris,
I just want to share with you the Star Ruler 2 Diplomacy. It's a unique and fresh diplomacy system vs others games. I don't think it's the best but it's really an innovative idea vs others.
I think the best will be yours off course haha
Here a screenshot:
http://media.moddb.com/images/games/1/35/34075/screenshot16.png You need to play the game to understand it but it's like a mini card game.
I just wish that you implement something different then a traditionnal "civ diplomacy system" or a "graphic civ diplomacy system". I think the 4X genre need an innovative new system.
If you find a good one it will help you very good for selling your game because it's the #1 thing in 4X game that every games is lacking.
I don't have the beta so I really can't have any ideas based on your game but we are brainstorming right
Others innovative ideas:
-With allies, you can create a Federation to overcome others players. You can be 2, 10 or whatever but it will counter the effect of a very strong player or AI. You can have multiple Federation. Think maybe of the WW1 or WW2 when everyone was on one side or the others, it was temporary but necessary.
-Council, like the real life ONU. Only very few can have a veto.
-Random events. I'm thinking about CK2 from Paradox that you can have problem when you force to mary another person in another country that can change the diplomacy stuff. Or maybe in CIV4 when you have all the addon... the random events can do great stuff in your advantage or can be very bad... I have no ideas what the Random events can be because I don't have the beta yet.
-I don't know if some ressources are limited but you can have something like to ally with someone for only this type of ressource and share the income or the benefit with them. Like in real life when 3 corporates work together temporary to build a bridge and after that they are ennemy again.
-Disaster...like I said I never play your game but I will take the example of Sim City with a tornado. The neighbourhood can donate stuff to help you out because you helped them in the past for something very specific for them and now you need them to survive. If they refuse to help you out because you are dying other species or some of them will react negatively with them. If he declare war againt you to crush you out...well the species that didn't like it will declare war agaisnt them without any warning.
I hope you find something good here