The only suggestion I can think of is offering the player the option of starting at various stages of the knightly experience.
Regardless of what stage you start in, you can progress through the others, forward or backward, as your fortunes dictate.
For example, the earliest stage would be a tutorial section where you're a squire.
Then there's a freelance mercenary who doesn't own any property. He could become a pioneer on the Far Marches or he could try to maneuver in the main kingdom for land grants. Or he could go to war if there's a war on. Or he could be a raider like the Danes were.
Then there's the settled-down landowner with his little castle or manor or whatever. Dealing with farming and political stuff and occasionally being called to war.
Then a more political high-level stage where you're a titled noble, possibly interacting with the royal court.
You could even say that the player has to unlock the ability to start at a higher level of play by actually playing through to that point. He has to play a squire at some point and finish the tutorial, for example, if he wants to start a new game as a mercenary. Or he could just play the squire character and try to become a mercenary.
I'm envisioning that the player can do things hands-on or he can delegate. For example he can sit there planting seeds and harvesting crops on his own, and so he can play the game as a farmer simulator. Conflict will still sweep through, but without fighting skills or equipment, or retainers, or advance news of the movements of the enemy, you're at the whims of the tide of war.
Or he could delegate, hiring officers to manage in his stead. The player could, from a map view, order constructions and fences and planting arrangements. And these skilled officers could suggest things, like "we need more training for our troops" or "we should be planting wheat around this time". Or the player could set things so a senechal just green-lights any of these decisions, so the manorial lands operate automatically.
New territory could be gained by buying it or conquering it. The King probably hates it when his knights kill each other off. Ideally you'd buy the land. But why would he sell? Only if he was suffering some great trouble and HAD to sell his land. And here the political maneuvering comes into play.
You can sabotage your neighbor, so he needs to sell. But then everyone else around has the chance to buy. So you want to position yourself as the buyer beforehand to maximize your chances. Or you put pressure upon him from above, lowering his status with his superiors. You whisper foul things about him into the right ears, and you snatch up his land after his downfall.
You can incite a peasant revolt, and swoop in to secure the property, and then buy it later for legal reasons. But the peasants need to be treated well for a long time because they are already in a revolutionary mindset because you put them there in the first place.
You could also marry properly, and marry off your children, in such a way that you secure inheritances.
I think just having the player travel with his ever-enlarging army, conquering manor after manor, is ridiculous. The King would raise an army consisting of everyone else and smash you immediately.
Oh, and no quicktime events. If you include NPC escorting, the NPCs need to be smart enough to never ever get hung up on the geometry and they must be able to keep up with the PC. Create a simple, non-exploitable economy. Include luxuries for the player to purchase, and keep the costs of construction high, and prevent attempts at industrialization. Don't let the player get complacent - stir the pot regularly - but don't turn it into a continuous struggle like The Sims or Haven and Hearth.