I don't know that I would aim for a main quest type experience, although I think that could definitely work. A few other things off the top of my head:
- Factions: Bump the noble houses up to 5, and throw apolitical groups like trade guilds or religious orders into the mix as well. Give the player the option to join a faction and eventually take it over, or even create a faction of their own. Then when civil war starts, alliances are drawn, betrayals happen, and ultimately it culminates in a big final battle for all the marbles.
- Basebuilding: The option to build a fort would've been pretty sweet. Maybe even grow a town up around it that gives you tributes of food and money.
- Enemy factions and leaders: More character for the bandits, orcs, undead, and other villains in the world. Make enemy leaders persistent within the game world. Give them bounties for the player to pursue that take place across a span of battles. Give them some reactive dialogue so that if run away from them, or they kill some of your bros, they will berate you for it. The world needs good villains.
Purpose and reactivity are the goals. Combat is fun but it's better when you have a strong reason to fight and when you can see that what you did made a difference (for good or ill).