Man, having mod tools come with the game would be so cool. Ooo ooo, I just thought of something:

Factional traits (i.e. Culture). You know how your king and heirs can gain vices or virtues through their actions, but they eventually die, and maybe only some remnant of certain behaviors lives on in their heirs. Well what if each faction (basically, you) had a "culture" that developed vices and virtues over time, based on the decisions you (the player) made. These characteristics would carry on throughout the length of your campaign; they would not die with your current king. You could change them over time if you got vices you didn't like, but it should take as long to undo a vice as it took to get that vice. Now I think that would be pretty badass.