Just thought I'd add a suggestion that'll hopefully be looked at by the developers.

My biggest issue with AI diplomacy is that they never really seemed too deterred when being threatened by a vastly superior enemy. For instance, I had the Altarians surrounded and I offered a peace treaty in exchange for all of their technologies. They didn't accept. A few turns later, they surrendered to the Iconians to get back at me, I guess.

Now, some may argue that they'd rather die than let their technologies be taken by the enemy; especially since with the current game mechanics, we can just declare war again instantly and take everything else after we took the technology.

So here is my suggestion: why not have something like a permanent vassal-state agreement? This was in Alpha Centauri (best AI diplomacy, period) and I thought it was genious.

The way it can work is that if you have someone beat, or if you have a neighbor who is about to get defeated and desperately needs help, they can become a permanent ally and constantly give you any technology they research and will basically agree to anything you say. They may harbor hostility and declare independence later when you're distracted, or they may just stay loyal to you in your moment of weakness. This could depend on their AI personality, their ethics compared to your ethics, other factors, etc.

This seems like a better option for someone than to just surrender to another person and become part of another empire. I think most of the civilizations would prefer to stay somewhat autonomous and just give tribute than to completely surrender their empire to a foreign civilization.

Just my two cents. The diplomacy AI is great for the most part, I just wish there were a few more options and that the AI wasn't so stubborn sometimes. Let me know what you guys think.