I just finished my game and diplomacy was disappointing. Most AIs were aggressive/violent, but I think it's because of the scenario so ok.

The grievance system looked interesting in the beginning but it quickly took too much space. I had a new grievance on my side or on th AI side every turn, and it just destroys the diplomacy. Because the grievance are either useless (the AI never accepts it) or the cause of a war. 

I felt like alliances weren't useful, border control neither.

Moreover, I had a bug : I was at war with an AI and at some point the war status just disappeared while i was still able to attack it (and it could fight back). In the diplomacy screen there was no longer any mention of the war and there was none of the buttons "declare war" "surprise attack" "propose peace" "surrender" "make alliance". I couldn't end the war and couldn't start a new one either.

The-Cat-o-Nine-Tales wrote:
However, that does not mean such suggestions are not valuable, as they will give us great insight into what you consider important in diplomacy, war declaration and resolution, and interactions with independent people.

So:
I like the idea of how humankind treats diplomacy, along with how wars are more of a means of getting the enemy to do what I want, however:
It does not seem like there are much options for me with interacting diplomatically with other empires, outside of war, and in a manner that does not inevitably lead to war. 

Political soft power options, if you will.

Like:

• I would like the ability to support a weaker player through providing them access to weapons/units they wouldn't normally be able to build (Having a treaty that gives another player access to building units above their technology level)
• I would like the ability to help build up a weaker player's infrastructure
• I would like the ability to have a treaty between another player/alliance member that allows for buildings to be cheaper built
• I would like the ability to impose stability penalties on other warring/snowballing players at a cost of my influence
• I would like the ability to...

• The mid-game becomes more of waiting for the right stuff to be built, with little interactions between players and my own empire outside of war.

• Make a no-man's land between the border of my empire and another player's empire, where no expansion between both of us is allowed in that neutral territory
• Make war where the goal isn't to take another player's cities and stuff, but is to take their influence production or even science production for my own.
• Make small scale war where the goal is to pillage things and fund my empire through more hostile means, without taking cities. Maybe even receiving a temporary war only bonus to ransacking for the duration of the war.
• Declare war on the behalf of a vassal that is under control of another player

Two things with the independent people:

First, they become incredibly oppressive if there's two or more aggressive ones nearby and there's no real option to deal with them without taking some kind of diplomatic hit or, if you are the one to engage, a loss of all your influence on them. I had to game the system a bit by saving up some gold and just buying up the aggressive units that came at me. There's gotta be some kind of third way to deal with them. Maybe urging them to attack someone else?

Second, if I'm happy with the amount of cities I have and don't want to assimilate an aggressive city, but also don't want to be attacked by an aggressive people, I have very few options (one being the strategy I outlined above). Maybe if you have above a certain amount of influence on them, they become less aggressive toward you? Maybe they can use some internal counter for how much money and influence you've used on their city to determine whether or not they should ransack your resources, luxuries, administrative centers, etc.?

My final thoughts on diplomacy.

I think the personality traits are great and *probably* change how each AI plays the game.

However, I did not notice any difference in the actual diplomacy screen. Every AI acts/reacts the same. There is always the same progression of treaties. They will never accept one treaty before accepting another one (first one is luxury trade). Once you get all treaties, then it's automatic they will accept alliance. Once accept alliance, they will always offer the middle two choices first, and sometimes forget about the other two. But they will always accept them.

It might not be exactly how I described, but my main point is: interacting with the AI in the diplomacy screen seems to have no differentiation regardless of what their personality is.

There is a pretty significant (and likely unintended) exploit of sorts that should probably be addressed before release regarding war. Right now when a war ends, all units that are still inside enemy borders immediately are considered to be "trespassing". This generates grievances which can be used to start new wars very quickly that you otherwise would not be able to, but more disturbingly allows you to immediately exit any cities that were being sieged, surround an enemy army, and completely wipe them out with weak units by fighting them one at a time. I blatantly abused this mechanic on a few of my highest difficulty runs.

My suggestions to help with this issue are to either flag the unit as "in exile" similar to EU4, or immediately teleport units out of the territory similar to in civilization. Alternatively, you could just give a 5-10 turn "grace period" where new war can be declared and no grievances are generated. This also would give the loser a bit more time to recover.

As for independent people, it was possible to hire mercenaries from independent people and use these mercenaries to destroy their own city. Although it was quite fun, it was embarrassing and unrealistic... Mercenaries destroying their own home? I'm sure it might have happen sometime in our history, but not on a regular basis like this. Here are some screenshots from my gameplay.

O
O after renting army

O Siege of Luluban

O after the fall of Luluban, the units remain unchaged, until their rent period is expired. Once expired they return to their original status, but with modifier "dead" instead of "in decline"

I think at least hiring enemy to kill enemy's city should not be possible at all.

I feel like seizing territories and vassalizing from surrender after wars should cost influence on top of war score. It currently costs influence to claim territory when it's open early on, but no influence to take when it's already occupied by another player, which seems a bit off. This would also give players something to do with their stockpiles of influence when its use diminishes after the early game.

The city still have the status occupied but ihm no longer at war with anyone 
Its the city of a vassal nation but after i defeated their leader, the nation was free but the city still occupied

I am very concerned with the fact that you get more money vassalizing someone than you do if you anihilate them and administer their cities yourself. It's mind boggling, because the difference is about 1k gold more that you are getting from vassalage and capturing every single city is much more costly in terms of effort and war score

Later in the game (after all nearby territories were claimed/settled by major powers) I was being attacked by seemingly random groups of independent peoples.  I couldn't figure out where they were coming from.  This should be made more clear to the player (is there a way to stop them from appearing?  is that something the player can control?  Can you predict where they will spawn so as to add extra defenses there?).

Overall, diplomacy, war, and independent people were pretty good.  I do think a "skip unit movement animation" option should be available (if there is one, I couldn't find it).  Managing many units with pre-determined paths would take a while just to wait for them to finish moving, and took the most time out of each turn.  Also, if a unit path requires too much movement, it shouldn't require me to hit the "skip turn" button, it should recognize that from the set path.  The usual example was embarking/disembarking units, which seemed to take a full turn.  If my unit had even 1 movement point left, it would force me to skip their turn, which would cause my pre-set path to cancel.

Diplomacy was fine, except that being asked for trade agreements that I have been "countering" every turn would get a little annoying, just because I'd have to manage it each turn and it would open up a separate window to manage.  Honestly, I'm not even sure what an ideal fix for that would be, except to make the process shorter/simpler (which I'm not sure I'd want) or to make AI ask less often.

Independent people were okay, but they mostly just took up space.  Even so, that IS kind of their purpose, so that works.  I don't like that assimilating them creates a city, since it can't be downgraded to an outpost (at least early on).  That is because early-game city cap is harsh (rightly so).  One possible solution (to support the idea of "independent" people) could be to allow establishing them as a protectorate (or similar) rather than assimilate.  Some kind of formal protection that at the very least discourages others from taking them.  There is the demand you can make AFTER they have been assimilated by another, but I'd like something a little more preventative.

Lastly, I did encounter one weird behaviour (bug?) when trying to start a siege on a city.  Essentially, there was only one path to the main city and the route was blocked for some reason to my units.  I couldn't move through one of the hexes between us, even to start a siege.  I ended up being able to start the siege by attacking an outlying district, but it was really confusing and put me in a slightly worse position.  I'm guessing it has to do with starting a siege on specific districts, as the land was traversable mid-combat.  Pic included.

I don't understand how the 'Heressy' grievance works. The black AI triggered it against me and I refused it and she had to back up because I was more powerful. But she then triggered it again and again and kept building war support every time I refused it until she managed to declare war with a hundred war support in 3 or 4 turns. Why is that grievance not extinguished once you trigger it? Why could she trigger it multiple times? I don't know if this is intended but, if it is, it should be better explained. Also she was able to trigger it while we were at war and kept getting the +5 from grievance every turn, which compensated for some of her lost war support in defeats.

I was forced to let enemies surrender. I'd rather kill them out completely, even if it affects my reputation very negatively.

Overall, I am unable to decipher anything from the displayed traits of other governments, nor do they seem to effect much.

Grievances seem pointless, no one ever pays the and I never pay them, and it seems to do nothing.  

There was once instance where I let a bunch of refugees join a city, but I don't think numerically they ever did. But I do like the idea of refugees.

Outposts are good idea, I'm not entirely sure of their functionality, but they should be able to spontaneously become a city without penalty, IF they are happy enough. 

Several things :

1. It's really hard to keep good relationships with other empires. There are always grievances that come to destabilize it.
2. Grievances should be less frequent (at the end of my game, I had one different grievance each turn against everyone) but more restrictive. It should force the other to take a decision, instead of just leave it to be.
3. Weaker empires should be more hesitant to push grievances, for fear of a war they can't win.
4. I didn't understand the war support loss system. I may be wrong (cause it's not really clear), but I had the impression that my enemies lost as much war support by retreating a 1-scout army than by getting a 5-units army exterminated. If that's the case it doesn't make sense. Also losing a province, having a city besieged or getting a building/quarter ransacked doesn't trigger war support loss ?
5. War resolution system is interesting but is lacking options. Also an empire should be able to ask for peace before being completely defeated, to try and save what's remaining.
6. I didn't understand the influence of personnality on diplomacy.
7. Diplomatic options are lacking. I'd have love to have the possibility of declaring joint wars, joining existing ones to prevent my enemies to gain too much influence, stop tolerating squirmishes without complete passing on the borders, asking for tribute by intimidation... Or just setting myself the amount of gold I want to ask when I counter a treaty proposal.
8. AI are ALWAYS at war. Which leads some of them to have completely incoherent borders.

[BUG?]  Independent people.   I was able to assimilate an independent tribe that I had ZERO influence on the meter.  Another nation that I'm allies with was 100% and then then it allowed me to assimilate them.  Bug or by design?

I was reasonably satisfied with the diplomacy & war mechanics, though I think the war support losses/gains should be tweaked to scale relative to the size of the battles taking place. Forcing a single scout to retreat shouldn't be as worthwhile as crushing an army of 8 swordsmen; it just results in wars that the AI give up on too easily and quickly without proper combat.

Independent peoples seemed somewhat interesting as a source for mercenaries & later assimilation, I didn't notice too much changed from Lucy but I've no complaints about them.

Things I found.   Diplomacy is in short supply for some cultures early on and greatly decreases the success of that player later in the game.  Was not as bad on the larger map as it was in past plays but still evident.  

All governors seem to have some negative trait that would lead them to be something akin to mass murders (granted that is how most early power was taken) but that should change somewhat as the cultures develop.  This being said, it did not seem like the traits had very much to do with the personalities.  

Wars,  I think there should be more flexibility on whether peace is taken as well as the turns of the peace.  I was forced into taking peace and even though I was the winner of the war, I had very little say in what the terms were.   I was forced to take things that did not want and I was not allow things that I did want.   No options for early peace.  No options to decline peace.

Grievances. - Do not really see a need for these is the cultures will never pay out  or force the issue.  I made demands and they always ignored them (They did not even refuse them.)  They made demands, I refused them and they always pulled them back.  

Borders - one sided.   I am not allowed to travel into their borders they ignore my borders and travel where ever they like.  (results See grievances above).  If borders are in place it should be enforced for all the same (players and AI). 

Treaties - AI do not seem to care much about them.  Also they seem to be stuck in a cycle many times.  They offer, you refuse, the offer the exact same thing.  I had this happen 10 times in a row.  They also do not seem to follow the the treaties set even when they offer the treaties themselves. Ex offer non-aggression then come thru your borders of a city and attack a troop inside your border and no war is declared or triggered

Things I like.   The differences of Outpost versus Cities.  The fact the independent states followed the same process.  The assimilation of the independent cities (or not) was a nice touch.  I liked that fact that if left alone they would eventually just fade away. 

Combat does seem more balanced and refined now versus in the past.