Have some diplomatic ideas ide thought i would throw out there for some feedback.

Non Agression Pact - With this you can basically ensure peace between yourself and another nation, even if they are annoyed with you. It basically allows two nations with a less than cordial relationship to be able to guarantee peace, for the duration of the deal. Might come in handy if you want to declare war on your pactees friend, or perhaps to give more breathing space whilst you improve relations with someone else to get a defence pact.

Loans - You ask for a loan from a nation and you pay it back over 10,20 or however many turns - with added interest. The lender dictates the price of the interest rate. Perhaps nations who you are friendly with can give a lower interest rate. Or perhaps strength of lenders economy could dictate interest rate - i.e. a nation with strong economy will charge much lower interest rates.

Supply Arms - You or the AI could ask another nation to gift it units to help it in a war. i.e - saladin asks me to gift him 10 units( you choose the units) in his defence against the mighty catherine - You automatically gain a + in diplomacy from saladin and a - in diplo from catherine( but at least you havent had to declare war)

Create economic bloc - This would involve getting friendly nations to join your economic 'club' . This basically allows first dibs at any trade or tech deals to nations within that group. If no tech or trade deal can be reached on a particular new tech or resource then, and only then, it can be sold to nations not in the group.

Thats it, they are most likely full of holes and faults but i really think this game could benefit from a lot more interesting diplo options.

Co-signed.

There should DEFINITELY be a lend-lease option...I think that SO many posters have said the same thing that I wonder why Firaxis didn't add this long time ago.

And yeah, I agree with loans...also, I think they should inflate how much each country has...it looks dumb how you're talking to a super-power empire and they can only trade 2 gold/turn. If you multiply this by a 100 for everyone, then you can have more denominations, and options as to how much you want to give.

The economic bloc is awesome...there should also be organizations like NATO, WARSAW pact, etc...no need to name em those things, but there are clubs you can create and even name them whatever you want to....and the AI should pressure you to join their respective blocs...

I like most if not all the ideas above. Diplomacy in civ 4 is already good but still has room for improvement. another idea is forming alliences between multiple civs and you can name it, have a leader, take action ets. Only ideas.:)