There should not just be one, singular campaign quest.
I feel the best way to solve the mid to late game slump would be to have added triggers that unlock new campaigns/events.
The only campaign that exists is one where you form your own kingdom.....I was going to do that anyway why is this the only campaign???
The triggers should be linked to various elements of the game and your progressio. When you/your clan/your kingdom have between X-Y number of fiefs/towns/units/clan tiers/companions, than Z happens.
This could have been an invading force/crusade, the introduction of ships and sea transport, the blocking of passages, a drought across most of the land, a bolstering of a single faction which gives them more power and troops from across to sea, making them a more formidable foe (and a million more that I'm sure others can think of). You could have mixed the game up a lil, especially for the mid-late game, and if the player didn't want any of these events, then they could just play Sandbox mode.. Its reskinned Warband with less content and I don't know how you managed to do that.
TW, you had umpting years to come up with something better than a mechanic that is already in the game. Hell you could have held back on the introduction of particular siege engines until a certain time period has passed. You could have asked the community, "What campaign's or events would you like to see in the game as we are struggling to keep the player entertained in a vanilla playthrough after they form their own kingdom."
whomp whomp whomp