I never understood the appeal of founding your own kingdom. Sure, I get that it's possible and something to aim for but the amount of work that's needed to get one up and running always made it feel (to me at least) more trouble than it's worth.
To be fair, my personal aspiration in most M&B playthroughs is to become a powerful lord with a city, a spouse and a high-tier army. The way I see it is that this is pretty doable set of goals that can be reached in a reasonable amount of time but with enough variation to make things interesting. What Bannerlord seeks to alter with this equation is the possibility that, after you do all that, you can be succeeded by an heir who you might decide is interested in becoming a King or Queen in their own right.
Then again, setting up a Kingdom in the beginning might be easier than trying long after the late-game when your chose faction could already painted the map with their colors. So either new mechanics or mods would need to be brought in to make this a more viable option.