Okay, so I haven't started a new thread in a while, let alone a brainstorming one. This should be quite a simple one to complete. Basically, here we just suggest ideas for different option a player has when entering the diplomacy screen with another nation. Try not to copy other games too much, and try to keep the list concise. For each suggestion, give it a name, and a description of what it does. Put the name of the suggestion in cyan so it is easily distinguishable from the description. Here are my suggestions:

Send Ultimatum: Send the target nation a list of demands. If the demands are not met, war will be declared. Either way, decreases relations with the country.
Royal Marriage: Propose a marriage between royal members of your two nations. Increases relations if the country accepts. Only possible with monarchies.
Offer/Ask for Map Information: Offer/ask for the nation's map information. Increases relations if the country accepts, as well as updating your map.
Send Gift: Send a highlighted resource or object as a gift to another nation. Increases relations if the country accepts.
Send Insult: Send a cleverly worded insult to the target nation. Reduces relations.
Offer Alliance: Ask for a military alliance with the nation. Increases relations if the country accepts.
Sue for Peace: Offer to sign a peace treaty with the nation.
Offer/Repeal/Ask for Open Borders: Offer/repeal/ask for access to land owned by the nation. Increases relations if the country accepts.
Sell/Buy City: Offer to sell/buy the target city/cities. Increases relations if the country accepts.
Request/Give Construction Rights: Request/give the rights to be able to build buildings on the target's land.
Demand Political/Religious Conversion: Demand the target country convert to your religion or government structure.
Annul Treaties: Demand the country to annul all treaties signed with a target nation.
Request/Give Research: Request/give the country the selected Researches.
Offer/Ask for Loan: Same as the "Offer/Ask for Resources", but this time there is a set interest rate to be repaid, along with a repayment of the original sum.
Declare War: Plainly declare war on the country, without any offer of surrender.
Offer Unification: Offer the target nation a mutual unification of the two countries. Very hard to achieve.
Ask Nation to Attack: Ask the nation to attack the target nation.
Offer/Ask for Resources: Offer or ask for the selected resources/tech objects from the nation.
Declare Ceasefire: Calls a temporary cease to hostilities between the two nations.

Claim Land: Requests ownership of land from bordering countries. More likely to be successful with friendly countries, but decreases relations. In times of war, severely decreases relations.
Declare War: Sends a nice basket of cookies Official recognition of a state of war, allies may assist the involved countries in the conflict, and alliances are more likely to form between similar peaceful nations (i.e. nations holding similar beliefs relevant to the conflict).
Request Unity: Request that the countries merge political systems, to form a United Kingdom, United States, etc.. May eventually merge to become a country.
Request Military Assistance: Request military assistance in any ongoing wars.
Adopt Monetary Currency: Adopt another country's currency.
Request Material Assistance: In times of major crisis, request support in the form of food, materials, and buildings. Decreases relations slightly, depending on quantity of materials required.
Request Governmental Assistance: In times of major crisis, request support in the form of temporary governing and order-keeping of the country. High risk of takeover from more unfriendly countries for the duration of the governing, but if successful increases relations.

I really don't know much on this subject, so I expect many of the descriptions have inaccuracies, for which I apologise. At least a few of them should be useable.

Don't worry, your suggestions are great. Now, to address each one...

Rejections in orange
Revisions in yellow
Approvals in green

Claim Land: The thing is, when you purchase or demand a city from a nation, the land around it automatically becomes yours. Therefore, land can already be claimed by sending an ultimatum or asking to buy a city.
Declare War: Well the idea was that this was just a possible outcome of sending an ultimatum, but for the nations that really just want to get to the fighting, I guess we could include it.
Offer Unification: Good one, although this should be hard to achieve to prevent countries just spamming this to take over the world. The player would then take control of the new, unified country.
Ask Nation to Attack: I recommend modifying it to this, so that not only could you ask a country to help you during a war, but at any time you could ask a country to attack someone else.
Adopt Monetary Currency: I don't see this as a diplomatic option. If the player desires to change their currency to loaves of bread, like the country beside them, that country doesn't have to give them permission, they can just do it anyways.
Request Material Assistance: I wouldn't include this one, in favour of the one I have listed after.
Offer/Ask for Resources: Offer or ask for the selected resources/tech objects from the nation. (This way it is not just confined to times of war.)
Request Government Assistance: To put it short, very hard to simulate. Interesting idea though.

And to add in one of my own ideas:

Offer/Ask for Loan: Same as the "Offer/Ask for Resources", but this time there is a set interest rate to be repaid, along with a repayment of the original sum.

I had another one but I forgot.

Ok, after doing some research into older threads on this topic, I got a few more, along with some of my own ideas.

Request/Give Construction Rights: Request/give the rights to be able to build buildings on the target's land. (Should I add this one? I'm not sure.)
Demand Political/Religious Conversion: Demand the target country convert to your religion or government structure.
Annul Treaties: Demand the country to annul all treaties signed with a target nation.
Request/Give Research: Request/give the country the selected Researches.

Offer Unification: I think this one needs some tweaking. The player and/or AI could simply abuse it, as mentioned before. Here I've got two ideas on this diplomatic move:

1. If there is actually some sort of hereditary or dynastic system in the game, unions should go like this (the idea is taken from Europa Universalis) : This option would be unavailable/grayed out for most nations. The nations it would be available to offer to would be the ones you have a personal union with. A personal union in real life is when two or more independent countries are controlled by the same monarch/ruler. For example, up until the treaty of Lublin, Poland and Lithuania were to independent states, even though they were both run by the same ruler. The way personal unions would be created is: a) If a monarch of one nation dies without a legal heir, there are three outcomes. One is that a noble from the nation takes position of the monarch, and the country takes a stability hit (a new king spawns).Two is that, if the nation has a very low stability, there is a medium chance that a revolution occurs. A revolution can either be violent or non-violent. There is no fighting in non-violent revolutions, and violent revolutions leave the state in anarchy until either party wins. The third is probably the rarest- it can form a personal union with a random nation it has a royal marriage with (although those with a higher influence and administration have a better chance). When in a personal union, the selected nation cannot declare wars on everybody except for their overlord, cannot form alliances, cannot have royal marriages etc. They are also automatically allied with their overlord. The "overlord", or the nation the monarch originally came from can offer the junior partner a unification. Different factors would be taken into account, and the junior partner can either reject (decreases relations) or accept (overlord inherits the junior). b) Through peace treaty.

2. If there won't be a hereditary system, than the requirements should look something like this:
1. very high relations
2. Military alliance
3. Royal marriage (if both countries are monarchies)
4. The proposing nation has a large influence

The second one.

Here is a few ideas, but I am not sure about it:

Declare Limited War:When you declare a "limited war" on a nation, it is similar as declaring an ordinary war, but you cannot do some things you can in an ordinary war. In a limited war you can: Use strategic warfare (bombing raids,convoy raids,countryside pillaging etc.) and skirmishes (battles) are also possible. However, you may not take enemy cities and you may not demand territories in a peace treaty.
Offer Ceasefire/Truce:The difference between a ceasefire/truce and an actual peace treaty is that when a ceasefire/truce takes effect between you and the country you are at war with, you cannot attack them, however, you:
1.Cannot demand or offer anything when purposing a ceasefire. You can just offer it, and that is it.
2.Can break the ceasefire, but you will instantly get a huge stability and happiness hit. Your relations with the targeted nations will also be severely damaged.
3.Will still be in a state of war with the targeted nation, but you cannot attack them. Once the truce expires, or is broken, both parties can resume with the fighting.

The second one is good, and I would add a variable time for the truce. However, the first one can be easily merged into regular wars. There's really nothing that should stop the player from taking the enemy's cities if he wishes, and so I would suggest keeping it as just a single type of declaring war, but allowing the players and the AI to fight the wars as they wish. The war they fight might just happen to be play out as a limited war, or it might happen to escalate into total war. Also, unlike EU3, players can attack other nations without declaring war, but war will automatically be declared upon the attack.

What about cold war?

Cold wars would naturally arise during gameplay, and would hence not require coding. For example, in my recent game of Civilization V, I was on hostile relations with Siam, we were both building up our armies and strengthening our shared and tense borders, and we were both wresting for dominance over the same city states. However, we were never in a state of war. it wouldnt make sense if I wouldve had to enter into diplomacy with Siam and picked a button saying "Cold War" just to do all those things. Cold Wars are basically comprised of non-military, hostile competitions between countries.

Offer non-aggression pact:(this will be relatively detailed) Offer another nation to sign a non-aggression pact. The chance of the nation accepting the offer depends on:

1. You relations with them- Good relations would drastically increase the odds of the nation accepting. Bad relations wouldn't mean you have a very small chance to get the non aggression pact signed- bad relations have a much less effect on the pact than good relations do.
2. How much influence do you exert- The more influence, the better
3. Their war exhaustion- If there is going to be war exhaustion, than the nation with more war exhaustion is more likely to accept the deal.

If the non-aggression pact is signed, the treaty will get it's expiration time, or how long it lasts. If the non-aggression pact is broken before it expires, the attacking nation gets a stability hit. Also, relations with other nations would be changed as this:
1.The defending nation- Huge drop in relations, can be easily turned from best friend to a deadly enemy
2.The defending nation's allies- Relations would get damaged very heavily
3.Nations who have good relations with the defending nation, or have bad relations with you- Relations would not get as damaged as with allies, but the damage would still be relatively large.
4. Neutral nations (either have neutral relations with both parties, have bad relations with both or good relations with both) - Medium decrease in relations
5. Nations which have good relations with you or bad relations with the defending nation- Relatively small change in relations
6. Your allies: Very minor change in relations
7. Your allies which are already at war with the defending nation- None

Also, you can't sign a non-aggression pact:

1. With nations with certain cultural traits preventing it (complete neutrality, jingoism etc.)
2. With nations you are at war with (obviously)
3. With nations which are not of your species/race who have not yet discovered writing (or something similar)
4. With nations who are at war with one of your allies
5. If you have a certain cultural trait blocking it

Good one, but I would simplify things here and there. And I wouldnt make cultural traits a restriction for it.

Ask for Research Agreement: You and 1-42+ countires/nations/empires band together to learn or research a certain part of the research web after a certain amount of time. If agreed, increases relations with all countries involved. Costs X amount of currency to accept. At the end of the agreement's elapsed time, research points ( or whatever system we are using) will be added towards that certain research option.