marriage proposals, can force acceptance by use of a favor. Currently only a set of hardcoded diplomatic actions, e.g.

Allow scripting of the actions that can be influenced by favors.

Make the requirements to create Trade Post moddable to allow for example others government types to construct them or to forbid them for special types of republican government.

Make the spawning of new characters from the create a vassal function (right click on a barony holding) a fully moddable decision.

Add a difference between vertical and horizontal scale for the map border for custom-sized maps, instead of the two currently in the game (1:1 and 2:3 ratio, I believe?).

Ability to mod mapmodes (far-fetched, I know).

Ability to define how many 'trade post'/'secondary holding' slots provinces have.

Allow the game to use negative dates, making it more practical for mods situated in BC years.

Ability to play as unlanded characters(mercenaries, holy orders, host invasions).

Make moddable/scriptable something that is currently hardcoded in the engine:

Add a command for listing all active global flags and variables.

An add_consort/add_spouse X-ID Y-ID console command similar to the new add_lover console command, which can help a lot with mod testing and story creation.

Console command to print every scope, command, and condition to the log.

Ck2 add holding slot