Well, I'm guessing (I'm no AI programmer) that the problem with complex diplomacy is that it's fuzzier than other aspects of the game. An AI can be pretty damn good at tweaking an economy to be optimal, since it's just optimizing numbers. Diplomacy, on the other hand, is messy.
But nevertheless, it's always at the top of my list of desires. Perhaps that's why I like CK2 so much.
I haven't played CK2 but, from what I've seen on Youtube it places too much emphasis on diplomacy between noblemen within your own empire. I'm not too fond of the idea of having to bribe the Duke of York to defend York in my name.
I do however like the idea of being able to give/sell units, form larger multiparty alliances like NATO, negotiating demilitarized zones, having multi-empire negotiations, conducting joint wargames with allies to build ties, generate experience and, to intimidate rivals.
I would also like to be able to build embassy's and, consulates. Sign secret treaty's. Arms limitations treaty's.
This is not diplomacy related but, the idea of having biological and nano weapons would be nice. Spreading plague to the worlds of your enemy's would be a cool touch.
A lot of this stuff has been mentioned years ago now and, we still haven't seen it. Don't get me wrong...I love what we have I just don't know why we haven't been thrown a bone on some of this stuff...including artificial world construction, star destroying, wormholes, jumpgates etc...etc...
"Tho' much is taken, much abides; and though we are not now that strength which in old days moved Earth and Heaven; that which we are, we are; One equal temper of heroic hearts, made weak by time and fate, but strong in will." -Tennyson