Welp, first off, BETA ME ANYWAY, but here's my take on diplomatic stuffs.
1. People will always try to find a way to game 4X AI Diplomacy, it's really what it's there for. Maybe have a bit of unpredictably depending on the race in question.
2. AI's almost never handle territorial dealings strategically well, I'd definitely make it hard to gain any kind of ground via diplomacy unless you feel really comfortable with how your AI evaluates the land. (e.g. player grabs the land mainly so he has an invasion point of said AI, so he asks for a piece of land that the AI might consider garbage right near his territory).
3. If AI's can initiate diplomatic contact, limit how often they pop up at the global level (e.g. if Race #1 just contacted them, Race #2 shouldn't pester you on the next turn). You can have multiple leaders contact you on the same turn, and war declarations can still happen anytime and should still popup since they are important.
4. "The 4X Diplomacy Point System", seems almost every 4X with diplomacy has a way to show the player how many points their offer is worth and how much the AI's stuff is worth. Get rid of this. Make offers generalized and actually require negotiation. You should only have a general idea of what the AI's civ values (e.g. they have a shortage of food, you have lots, so it shows up as a shiny green dot as something potentially valuable).
When you make an offer to the AI, you shouldn't get them all angry unless the offer is outright ridiculous (e.g. give me your city and i'll give you 1 unit of food, even though you need 50 to survive this turn). The AI should evaluate the offer then counter-offer with something that is high for them, and you can risk lowering their offer and making them mad from there. As long as your counter offer isn't too much in your favor again the AI should evaluate and re-counteroffer if it's still not good, if your counter offer is the same or still really bad then they get mad.
5. AI's should become suspicious of large military buildups on their border (and be equally sensitive to doing this themselves). Even as allies an AI should expect SOME military presence on the borders, but if it starts getting abnormally large they should suspect an invasion (with the exception of allies close enough to have both mutual defense and mutual war targets).
6. Some other posts have been made to this effect, but I feel you need to be able to tell the AI's your general stances on some things. When the game starts you'll get to set your initial policies (Don't like people that border me, those with bigger military forces worry me, size difference bugs me) and also place tokens down where you feel you'll be expanding, so the AI's know to either stay away or to contest you immediately.
After the initial policy/territory setting, you can change them at will but to prevent rapid changing of policies and gaming the AI, when you change your stance it will take a few turns to take full effect, and during that time your old policies begin to decay as well.
7. AI Should remember wars and how well or bad they went as a factor in their wardecs. Also the player shouldn't be penalized on future diplomatic deals with an AI if the AI was the one that initiated war.
8. Diplomatic reputation should be a factor, the AI's should track if a player tends to honor or break any deals they make, but the player being known for honoring their deals should never be a major honeypot factor for the AI to prevent gaming the system. If the player is known to break deals though the upfront payment should drastically increase to compensate for any long term deal the player may make with an AI until this improves.
Well this is all I've got for now