Just for the sake of throwing ideas around... Not a particularly bright one, but one I thought of while driving today.
Mount & Blade (I know, has been pointed out) with a touch of Crusader Kings 2.
You play your character, whom you develop in an RPG-esque fashion (Do believe it should be significantly simplified from the M&B implementation, though), and can earn titles, lands,etc. much like you already can in M&B. The actual "Title system" should have mechanics closer to CK2 - characters have claims to titles, and may or may not actually own them at any one time. Claims are hereditary.
Characters have traits, which reflect how they think and act. Should be a fairly select list, with emphasis on determining their motivations and actions, not their stats. Developing relationships with NPCs becomes more of a possibility - creating webs of alliances, enmities, rivalries, etc.
If you do own something, you can access slightly more robust functions related to it - i.e.: build cities up, train armies remotely, send patrols to keep your lands safe, etc. Key here would be keeping it accessible and, again, simple. This is a second layer of complexity on top of an already complex game, after all.
Would need some kind of logic to make the construction system work, as these holdings would have to actually be rendered out in 3D. Possibly a "building blocks" kind of logic to allow bits and pieces of structure to be pieced together by the system.
You can marry and have children. You can interact with your children (Potentially teaching them, building a legacy for them, etc.). Time passes. Your character will eventually age and die. You then go on playing as a chosen descendant. The same happens all over the world, so eventually none of the original characters will still be around.
There are interactions and events and a more robust NPC AI to get that feeling of history happening around you. Titles will shift hands, nobles will shift allegiances, noble houses will rise and fall.
Would need to think of a different implementation of the "Overworld" concept currently in the game to allow for a bit more of interaction with the world. Having men scout and forage, making ambushes, falling prey to sickness or starvation, using the environment for fights (example: Waiting for the enemy across a river ford).