Forwarding design in Nation Diplomacy, here are some more ideas.
Send Payments Ability
Nations will be able to send payments to each other. Standard rules will apply.
Tribute
A daily fee paid by one country to another.
Voting processes will open to both countries for tribute to be established.
Limitations will apply to protect inactive countries from getting in debt from rogue-kings.
Trade Pacts
Peace status will be required
Trade pacts will be forged between nations.
They will grant bonuses on sales. The bonus will be given to all Dukes or higher of the nation signing the pact. This bonus will be perhaps 5% for all resources except Gold bars (1.25% for them).
Ports could be specified for increased diplomatic options.
Daily fee (optional) will be available (especially handy for nations that do not control any port).
Service Pacts
Just like trade pacts only that they will refer to all other port-specific title bonuses (ship building,crew hiring,etc).
Allied Countries
An alliance will grant by default the trade/service pact perks (no need to separately define them for allies).
Mutual Defense Pacts
These will be private (hidden from public). Peace status will be required.
This is a different pact from an alliance. Mutual Defense pacts will have specific objectives which will be activated under specific circumstances.
For example:
Bermuda and Tokelau could sign a Mutual defense pact for Neapolis and Aiora for 10M a day/maximum 20 days. Spain however decides to become aggressive and captures Neapolis. This will activate the Mutual Defense pact Bermuda had with Tokelau. For the next 20 says, Tokelau treasury will send 20M in aid to Bermuda.
Another example:
Bermuda and Tokelau sign a mutual defense pact against Spain. If Spain declares war against any of the two nations, then it will automatically enter the war with the other too. In case Tokelau declares against Spain though, then since the aggressor is one of the two nations that signed the pact, Bermuda will stay outside the war.
More types can be scripted (different objectives/triggers/rewards/support)
Vassal Countries
We could script a vassal system as well. Something along these lines which I will describe with an example:
Let's suppose that Mexico is a vassal country of Spain.
-Mexico has the obligation to follow Spain in all its wars (automatically declares in all enemies of Spain).
-Mexico pays a daily fee (non-expiring tribute) to Spain as long as they are vassals (most probably a % of the daily tax income).
-Mexico and Spain enjoy all the perks and statuses they would get if they were allied.
-Mexico has no diplomacy of its own. It cannot be declared a war. A war to spain needs to be declared instead (so it will then automatically enter the war as well).
-It cannot form any treaties on its own (ie defense pacts, trade pacts).
-Any pacts signed by Spain with other countries (ie trade pacts) do not apply to Mexico.
-It's king is considered a Spanish puppet (I am role-playing obviously)