This is a sandbox game, the story is almost always what you make of it. Such as it is the main storyline is usually a distraction; a subplot to be completed (or ignored) alongside the original and developing goals you define for yourself on each playthrough. For that reason I don't care for it and frankly find it annoying. I do hope they will release an alternate gamemode that removes it upon the games final release.
That said you could make the story more interesting, at the moment the narrative does not alter the games world until the very end, and when it does it feels rather arbitrary. You could have the world and the factions interact with the story a great deal more: Peasant uprisings in Vlandia, a religous schism amongst the Aserai, a changing climate reducing the once fertile steppe to barren tundra, forcing the Khuzait into a desperate advance westwards. You could do a lot, there are plenty of historical sources to draw inspiration from. Every condition would have its trigger, hinted at through dialogue so the player wouldn't unwittingly embark upon something he/she wasn't prepared for.
Maybe this is something to be explored more in various DLC's, but if it is then why bother with a story in the first place? I think a lot people would be prepared to pay for a more fleshed out campaign mode.
In my mind I keep comparing this game to Kenshi, another open world sandbox rpg. Kenshi does not have a direct storyline, but paradoxically, it is rich in both story and lore. Everything is explained passively, through seemingly spontanious dialogue with various characters and events that demonstrate the tense relationships between the various factions. There is no hand holding and the game does not point you in any particular direction, the story is there if you want it and if it doesn't interest you it won't get in your way.
Maybe the comparison is a slightly unfair one, Kenshi does have a heavy emphasis on exploration which lends itself to this kind of storytelling. But I do think Bannerlord would be better with a more passive story.