I think that the current diplomacy system still needs to be tweaked.
I have started an all-fast game on a huge map against hard AIs (unlocked teams of course).
While I had some fights initially, the game quickly evolved to a state where most AIs were happy with me.
I picked a single empire to fight, which quickly got 4 AIs to get happy with me as a collateral bonus to my actions. I by basically defending from another one's attacks, I got 2 more to befriend me by doing nothing.
I got peace treaties and all basic pacts with them all, and the other AIs were basically ganged upon by everone else.
I got the game to a state where 6 AIs were fighting each other, and I was allied with everyone. It certainly didn't feel hard to do, it was just a collateral accomplishment by my actions.
In this state, it's also easy (though slow) to colonize without fighting. AI is exceedingly slow to send a colonize vessel once a planet is razed. Since i was allied to everyone, I placidly watched the fight between the AIs for a planet, and stationed a colonize vessel close to the planet. as soon as it was razed, my vessel took it over. The AI can leave a gravwell uncolonized for several minutes before sending a vessel to it.
I think that it all needs to be more fleshed out. I have already suggested to have the possibility of bargaining, which s currently nonexistent. Military cooperation should also have more if's, as the current glaring exploit.
I think that military cooperation should be a treaty, and one hard to accomplish, since it's so crucial for the game. This would open the doors for setting military missions. Without a military cooperation treaty it would not be possible. Empire A (AI) should be able to plan long term and set an objective to 'colonize planet X', and correspondingly ask empire B to help. But if empire B sneakily colonizes first, this should be seen as treachery and carry a heavy relationship penalty.
The empires should also be aware of military actions against third parties. If A is allied to B and at war with C, and B is allied with both, A attacking C would carry a penalty to A. If there is a military treaty between B and C the penalty is worse. If C asks for help from B and B refuses, their relationship would also deteriorate.
I also think that without a military cooperation treaty, jumping ships to a gravwell of another empire should carry a penalty, even if both empires are at peace.
I would like to have the option of trade and diplomatic missions. 'Send a diplomatic envoy to planet X in Y minutes', for instance, or 'do a commercial embargo on empire A to pressure him to stop attacking me'. Or 'send a convoy of at least 500 metal and 500 crystal to planet Z'. That would give interesting uses for trade ports and refineries. I could have buttons to build convoys, each ship holding 100 of one resource type, and setting a target for it. the player would have to escort it (or not, but that would be risky). Some would be trade missions with a primarily monetary bonus, while others could be answers to distress calls to help colonizers. the player could spend the cash by himself to help the gravwell, or set it as a mission to another empire - in the case of success it would carry a diplomatic bonus.
Other trade/diplomatic missions could be 'help planet X colonizers to develop infrastructure to house more people'. Translating: ask another empire to pay for a planet upgrade in exchange for a relationship boost.
Another thing is that I just can't use tactical and logistic slots if there are leftover structures from an friendly race. I am A, and allied with B and at war with C. C attacked B and colonized one of his planets, and after I razed the planet, B seemed uninterested in reacquiring it. I would like to be able to help an ally 'liberate' a planet i.e. getting it back to him. I got fed up of waiting and colonized it, but then there were some structures from B, my ally, taking up several logistic slots, and I couldn't attack them without breaking the alliance, nor scuttle - and they were still taking up logistic slots. Maybe foreign structures could cost no slots, or would gradually deteriorate. As a last option, having structures on the gravwell would have an 'ownership' connotation, thus colonizing a gravwell with structures of an ally would carry a penalty, because I should have left it for him to retrieve for his empire.