I had a somewhat similar thought the other day. These NPC entities could be an interesting addition but without a need to focus on them at all if it is not interesting to the player.
Having a NPC noble at your estate could cost you something. Perhaps more for the "better" ones. They could have their own stats that would help the estate in which they reside - not much, but something that would be a small bonus. The NPC nobles would age naturally and have a chance of dying of old age. They could also be assassinated.
There could be a neat way of introducing marriage in a way that would actually work. You could prevent player marriages, but you could always have a NPC spouse. You would just need to get him/her from another family. Then you could have NPC children too. You could also start playing any of the nonmarried NPC nobles you have if you have room for a new character.
Or you could just create the NPC nobles as needed.
You could also make decisions about the education of the young nobles. Say you wanted to focus on swordfighting and bureaucracy for your son, so as the game years pass, he would grow stronger in those. When you at some point decide to make a player character of him, you would get bonuses to those stats (perhaps something like 25% to begin with). Then if you make another noble mad, and he in turn decides to assassinate your son, it would really hurt and more conflict would ensue.
Or if you had bred a really potent bride for your duke, but an infiltrator creates a scandal which makes her spoiled goods and unable to be married... much fun would ensue.
NPC family members could be an interesting way to create wars too. Infiltrators could murder NPC members at the estates.
