I think having a family is a great idea, as it implies an other way of playing.
Your sons and daughters could grow up and join you in battle, become vassals, marry; but they also could be captured and ransomed, it would be fun. It may be complicated to implement though.
Building castles would be great, and of course it should be costly, only in your territory as well, and maybe there could be an area around where you couldn't build. It should take a while to be built proportionally to the defense quality, size, and location. It could be alike for villages and cities. I like it but it's up to the devs to see if they want to add a bit of strategy like in the Age of Empires series,
or not.
I'm with you on those two ideas (even though I'm nobody), but for the rest think about the players who like to play it the hard way around. I'm one of those so.... Anyway great ideas. I hope it leads to something.
If anybody thinks so too, please do what you can.