Hello. This idea is beyond my modding know-how, so I am throwing it out there. This would be for CK2+.

Is there any way a mod can be produced that Denmark, Norway, and Iceland can have a unique building at their trade posts that can represent trade routes to colonies in Greenland and Vinland after the Leif Erikson event? Heck or just use Scandinavian as the prerequisite. Upgrading this "building" represents the colony expanding. The Vinland trade route representation would be more expensive than the Greenland one, and rare events can happen that will damage the building or reduce it to a lower level, or even wipe it out completely.

I say use the building representation as an idea as the off map empire is obviously being used by China, so it's the next best thing.