A few technical observations on diplomacy:

• Establishing tribute prohibits you from going to war with the other nation.  This is never discussed or talked about in any of the in-game text I've seen when establishing the tribute.  You simply no longer have the ability to go to war once tribute is started.  This is either a bug or should be made a more explicit portion of the tribute treaties.  

• I have occasionally had an issue where a nation was "too busy to talk to me right now" despite the fact that we were not at war.  Unfortunately, this means that I have no way to declare war on them.  They are essentially not allowing me to go to war with them by ignoring me.  Maybe a bug, I'm not sure, but if not definitely a bad mechanic.  I should always be able to declare war unless I'm in an active treaty that prohibits it.  Personally I think we should be allowed to violate treaties and declare war anyway (with the appropriate diplomatic consequences like all nations trusting you less), but I understand if the goal is for the diplomacy system to be simpler than that.  Either way, I should be able to go to war despite the fact that the enemy doesn't want to talk to me.  

A few subjective observations:

• Make international relations more obvious.  The UI does not provide an intuitive understanding of the overall international picture.  Think the old Civilization diplomacy screens where the head of each nation was displayed in a circle with multi-colored lines from each head to every other head depicting their treaties and their overall relationship (with a basic filtering system to declutter the image).  If I want to invest in the diplomacy part of the game and play the manipulative behind the scenes nation I need an easy way to see what tensions exist for me to utilize. 
• Most screens make excellent use of tool tips.  The diplomacy screen has none.  For example, save space by moving the "Relations With Us" information into a tool tip that pops up when you hover over the total relationship score.  
• I've always enjoyed the late game TBS establishment of a UN or Global Council of Whatever.  Seems to me like something similar would fit here and could make mid to late game diplomacy significantly more interesting.  It would be a great way to emphasize the Kingdom vs. Empire mechanic, as well as to let the personalities of individual Sovereigns show through a bit more.  There are a lot of things in the game that may make for good "global council rules" or whatever.  Or possibly have two councils, one for Kingdoms and one for Empires, where each council works to make sure their members are behaving like a proper Kingdom or Empire through binding rules and what have you.  

Since we know city building is getting the next big rework I think that leaves diplomacy as the weakest part of the game (assuming the new city system is done well).  I'm curious to hear what others think or what the team has planned in this area.