Not long ago Brad requested feedback on diplomacy, basically asking what diplomatic features from previous 4X games we most enjoyed. I'm not a founder and don't have early access. But, I still play GalCiv2 regularly and had a few thoughts about shortcomings in diplomacy that I hope won't be repeated in GalCiv3.
1) I was just playing a game where the Korath Clan declared war on me. Several turns later, the Altarians contacted me, and said something like "Your war against the Korath Clan has no merit. We declare war." Wtf, they attacked me first! Similarly, I've seen cases where the AI would say that "X is the victim of a war of aggression by Y, so we're helping X", but I suspect that actually it was the other way around because X had a much larger military compared to Y. Basically, I'm saying the AI should keep proper track of who starts wars, and view those who start wars (including other AI players, not just the human player!) with increased suspicion.
2) A common occurrence in GalCiv2 is that when one civilization is becoming very dominant, another will contact you and say "Have you been keeping an eye on X? We are concerned about their growing dominance" etc. But they never actually proposed a specific action. If they are going to reach out to you, they should put something on the table, like offer you money or a tech to attack the strong civilization.
3) Finally, an extremely common occurrence in GalCiv2 is get messages like "We have decided to go to war against X. A third party with a great deal of influence persuaded us to do so". Ok, fine. I take it that they were bribed to go to war. But the AI never tries to bribe the human player to go to war. In general, I feel it's very important that the AI players treat the human and other AIs the same way.