That.. Is actually a very good idea. Something like the skyrim workshop has for its mods. Simply adding them to their game.
It sounds like a really great plan, I'm not a programmer though, so I do not know what kind of obstacles there are with realizing this. It would be awesome if someone could produce a mission pack, for example x amount of new quests to be added in your singleplayer game (preferably even without having to sacrifice your save game). This would mean that no special items, persons, locations and effects would be added otherwise it would not be compatible.
Quests to hunt down a specific enemy lord.
Quests to kill certain troops.
Quests to succeed in tournaments as the champion of a faction in order to gain renown.
Quests to deliver weapons and armor to cities.
Quests to bring cattle or resources to villages.
Quests to help your companions with certain tasks.
Quests to bring iron or other resources to merchants so they could create weapons or armor for horses.
Quests to assassinate people on the streets, for example lay an ambush in a city to attack a lord in the streets.
Quests to capture fiefs or even more difficult, to capture fiefs and influence the politics in such a way that it is assigned to a certain lord.
That kind of quests.