I think there could be some good new diplomatic agreements.

Vassal:

You gain control of the nation's foreign policy and their military. The AI will still control what they build and their citizens and such. If a vassal is attacked, you must declare war on the attacker. Good for a defeated enemy you don't want to spend the time on capturing every last city or a weak enemy you don't want to spend the resources on.

Trade Routes

Besides the normal trading luxeries and resources you could open a trade route with a nation. It would increase the gpt for both civs and would improve relations. It is set up between both capitals via a road or sea. If either way is cut then the trade route is ended. Blockades requiring single ships in the game could work well(one ship within one square of the port).

Knowladge Share

When this is made, 10% of your science spending also goes to the other country and vice versa. You're not sending money to them, but say you're spending 300 gold on science, 30 of that gold is shared. Say they're spending 500 gold on science, they'll share 50. So now it's like you're spending 350 gold, and they're spending 530.

All of these would of course last the standerd 20 turns.

i love the vassal idea... sounds an awful lot like my confederacy idea...

http://forums.civfanatics.com/showthread.php?t=105115

I like 'vassal'. What would be the non-legitimate way to break it though? (they all have to have one, usually it's declaring war)

I like Vassal, but I think it should last until the vassal civ breaks it or until the 'lord' civ ends it.

i think it should be really tough for the vassal to break it...

The trade route is a good idea which I have not yet seen before.

The trade would be like a right of passage agreement for trade between the two nations. The trade ROP would allow trade for either a set price, per turn, or per volume (aka tarriffs) or no trade barriers (aka Free trade-NAFTA, EU, etc).

The science knowledge trade idea I oppose as suggested. It would lead to complications. Why would any civ give 10% of thier GDP (let's say resulting in 100 gpt for science) to another that could give again 10% (but in this case contributing 90 gpt for science). It doesn't make sense.

Another alternative is to make an agreement where each civ will contribute a set amt/turn and poll all those science resources together. The science tech pact would require more techs, as things would be discovered much faster. I would perfer to to have partial upgrade techs (Defensive Tank Tactics- improve tanks defending points, Calvary accelerator, etc) These could be automatic or as I perfer, require these trait upgrades.