I'd love if Bannerlord would have a more in depth lore side to it. It could be in the form of "books" (like in Kingdom Come Deliverance or Elder Scrolls), snippets in conversations (where you can eavesdrop interesting lore, or where an old enemy faction member scolds you out), maybe even have some unique monuments or historical buildings in cities that already show the lore themselves.
For quests, I don't know if lore specific quests would really work out. It stays a sandbox game, and the example that a "simple" side quest like "bring this loot there so we can make a Casus Belli against them" seems a bit far fetched. It's stuff you have to make up in your mind yourself, using common quests to achieve a goal lorewise. I could imagine a dynamic system in Bannerlord to "fix" this. For example, transporting goods, specifically contraband, to a place A where insurgents are living, brings down the relation of your faction with the faction where place A de jure is located. It's a dynamic system what I just described, where common side quests automatically have consequences to the factions that pertain to it. This spices up simple side quests, as they have consequences on a bigger scale, faction wise.
I am not so in favour of a specific story line, as I think Bannerlord should largely sandbox driven, but as said I love to have a deep lore behind the world of Calradia (I loved this about Elder Scrolls). It should be thought of how to intertwine this lore with the sandboxism.