It's not possible to have a perfect diplomatic system as it has conflict goals. First, AI needs to look as human as possible. Second, AI should act with it's own agenda. It should be predictable enough to make diplomacy game meaningful, it should be unpredictable enough to not allow fully manipulatable AI as in Civ 3 or 4. If you throw in multiplayer, it becomes a mess.
Some time ago on the forum I suggested a "public opinion" system, which could partially solve it. The general idea is - the civilization bias and current likes/dislikes is represented by "public opinion", which affects both AI and human players. Leaders could overcome it, i.e. by attacking civ with good relations, but with consequences like unhappiness. The system should be able to solve general problems of diplomacy system, but it's more complex.
One of additional bonuses of the system is a layer of espionage game. You may want to reduce your population opinion towards specific civilization before attacking them, but this action could be uncovered with espionage, making planning for war visible even if it comes within a human player head.
That's the basis of better diplomacy system I see
The risk with public opinion is that it becomes a mini-game in itself, unless it is integrated in with the growth limiter mechanic of the Civ game, be it happiness or health.
Civ5 AI especially in its final BNW form often surprises in its prescient read of the situation. I just wrapped up a Venice game where the AI made it as hard as possible for me to win, and I almost didn't win.
But to some players, the lack of tool tips means their default read on any AI action is assume the worst (incompetence/poor AI/psychopathy) but the AI follows a fairly easy to understand logic.
They appear to asses their VC and their rivals every 20-40 turns, just long enough for the Declarations of Friendships to expire, and so long as players understand friends today can be rivals tomorrow and friendships in Civ5 are contingent on shared interest as the game evolves, and not the bribe the human players dumped on the AI, then it all makes sense.
So in that context, a public opinion system would work if it is simply the readout for players to interact with the AI, to more clearly see their agendas and goals, rather than understanding them through interactions with the AI and knowing each individual AI leader's preferences
From the sounds of it, Civ6 will have more tooltips in the diplomacy screen and I'm all for that. I particularly want easy access to information trade deals, locations of cities in a minimap when discussing deals with AI leaders.
That said, I want there to be unspoken inferences to AI behavior that isn't 'told' to the players that players need to pick up from playing the game and knowing the leaders historically.
Losing the experience in learning the AI's agendas could alter the flavour of diplomacy from the current state of making an educated guess and planning your grand strategy around that guess and then discovering you were almost right or very wrong, vs. KNOWING the AI agenda inside and out and just playing with the numbers to manipulate the desired diplomatic result.
The latter isn't so much fun to me. That's not diplomacy. And that's a dumb down diplomacy for people who just want to min max numbers. They don't call diplomacy statecraft for nothing. It's as much an art as a science. There's a lot of relationship building, agenda seeking, and backstabbing involved.
