What diplomacy improvements are we looking for though? I think it would help if we gave specific feedback.
I can think of a few things....
1) A diplomacy/treaty summary screen ala GalCiv2 (https://www.galciv2.com/darkavatar/screenshots/final/index.28.jpg)
2) The option to close borders to a faction (possibly at the cost of a relationship penalty)
3) AI being smart enough to understand that maybe sucking up to the faction with the highest strength rating is not *always* the best idea, i.e. if the leading faction is starting to dominate, maybe weaker factions should form an alliance to try and stop them... particularly in the later game
4) Would be great be able to trade a City or Outpost
5) Could also have the option to trade map details, i.e. lift hidden tiles that the other faction has uncovered (Total War Medieval 2 does this, presumably the other TW games do also)
6) Gifts, tributes etc don't seem to have much an effect on diplomatic relationship... would be good if giving a faction a heap of money made them like me a bit more.
7) If a faction demands a tribute, and it is refused... what happens? Does it affect the relationship? I could be missing something but seems it doesn't have much of an effect.
8) AI needs to be better at gearing up for war, and sustaining a war... if it's going to declare war.
9) I think the faction strength ratings could use a bit of a tweak... particularly with regard to the armies a faction has, and their production capability. Lots of weak crappy units seems to really boost faction strength... but often the human player can wipe the floor with weak troops. I dunno not sure exactly about this but I think it needs a look at. Others would have better ideas on this than me