Two people voted other and didn't bother to explain themselves? Sad!
Of the options I think diplomacy is pretty self explanatory buuuut...
1) Multi-national agreements would be nice.
2) Less concrete trade negotiations. Instead of just plugging numbers to find the highest trade you can get it would be nice to have an "attitude" for the AI that starts with willingness to trade but can end up being insulted and won't deal with you no matter how generous the proposal. This new scale should not be static either. Proposing more than one deal that yields an "I doubt it" would increase the likely hood of your proposal being insulting. Of course you wouldn't get the trade advisor's messages until after you submitted a proposal so you'd always get one freebie. If your second proposal is still insulting I wouldn't expect to be able to make a third offer.
3) AI to AI diplomacy should not be automatically successful. The RNG should be utilized to add variation and realism to AI negotiations.