[Strategia_In_Ultima]

More in-depth and, more importantly, moddable diplomacy.

Say, you just encountered another race. You can only send General Messages until you've established some sort of consulate there, at which point you can propose the more basic exchanges and such, such as gifts, trades, trade treaties and such. Then, if you were to establish an embassy, you could start working on the more complex agreements, such as Military Alliances, joint war efforts, Surrender and so on. This wouldn't just be a Nothing-Consulate-Embassy system; this would work with numerical levels, so that Consulate is lvl2 and Embassy is lvl3. This way, you can mod in more diplomacy options, so that a basic Consulate would allow you to make gifts and non-aggression pacts, whereas a Trade Consulate would allow trades and trade treaties, Embassies would have even more complex treaties and Joint War Coordination Centers would allow military alliances and joint war efforts.

Also, I would like to see moddable treaties. This would allow for a more complex, in-depth diplomacy system, keyed to the specifics of a mod. And, mod specifics aside, it would give you a potentially vast amount of diplomatic agreements. I for one would like to add at least these following treaties;
Loan, Mutual War Effort, Vassalization, Forced Annexation, Merger of Empires, Trade Boycott, Mercenaries, Multilateral Alliance, several types of more in-depth Trade Agreements, Mutual Intelligence Coordination, Research Alliance, a bare-bones Military Alliance und so weiter.