Good I could be of a little help
interesting idea - kill a before named faction as player or as a vassal? - As a vassal would make it easier - especially if you are the vassal of a strong faction say Hungary against Holy Roman Empire......But it is an idea I like - I personally prefer to be a vassal in my games - especially when they are played on historical maps.
If you could make a list of nations you would be likely to add later I could make an effort in getting info, lists of lords, what towns, cities etc are most prominent in the period etc...It would also help other interested people in getting info collected - add it to a list on the first page perhaps.....- likely factions, out of the question factions etc etc Croatia seems the nation you are working on? - not really my field - Venezia? Italian city states? Venezia isn't really that strong in terms of area but one could combine some areas - turn them into leagues/alliances or the like - on the other hand, you could argue that it would be nice to have large factions and small in one game....as for the Teutonic order - there you could lean on the work of the Rus XIII mod....for lords etc.
What I am a bit unsure about is the map - from the screenshot the map would suggest that there would be room for the dutch provinces, most of Denmark, the Eidgenossenschaft (Swiss), Croatia, Flanders, Lorraine and more of France, Italian states, northern Serbia, Wallachia (if I remember correctly under the rule of mongol vassals or tribes at the time - Cuman?) What is there really room for? what is not useable of the map? - Would be nice to have a screenshot of the map with the actual borders of what areas can be used to figure out what makes sense in terms of research and suggestions...