Treaties would come in 3 forms, Peace Treaties, Trade Treaties, and alliance treaties.
Peace Treaties would be a treaty which 2 players wouldn't be able to attack each other for as many turns as they wish, this could be used as Aaron way to temporarily ally with players when they see a greater threat is among them.
Trade Treaties would be a treaty which allows the for one Nation to give support by giving troops to the other country, the trade treaty could be stopped at any time.
Alliance treaties would be a form of Peace treaty which allows a group of countries to work together by support and buy land from one another.
The can be accomplished using the new mod framework!
See the Advanced Diplomacyv2 and Gift Armies mods as well as Commerce setting.