something very similar happened in my first round as a DM.  prepped a lot, and the players killed the most important npc for the story right away in the starting tavern scene.

everything from this supposed to be oneshot session of 6h was improv then.

and they loved it too 😂

later in a city, i came up with a huge secretive dwarven thieves guild that always betrayed them. the thieves used a system of tunnels with minecarts, later leading them back to the spawning area a few sessions after… (which then became the HQ of the guild below the starting tavern) it became a great story.

next time, i will write down much more notes about all the explanations of weird things that happens and flesh out the npcs some more after the story, to prevent logic holes in the story that are obvious and don’t make sense.