Is there any mods out there that have fairly detailed companions who start out as your vassals or servants in your new stronghold? I think it'd be a great point to introduce someone with a storyline, and I'd be surprised if no one's taken the opportunity yet.

Ummm, Uvirith's Grave can have Gavyn Helerim as your apprentice in your tower he is a very powerful and very detailed companion, and Building up Uvirith's Grave adds a city and it can have fast eddie as a sort of "vassal" in it.