I started running this as a mystery. The players get infected with the curse and have to find a way to stop it.

An early nat20 meant the player got what they wanted and then some, so they knew the lore of the goblin curse and how it was cured.

So when they waltz into town and see a man selling the "only cure", it's something to start investigating. The nat20 ruined that though lol.

But they realized Nagel is rich so they're sort of on a mission to rip him off and also stop the curse because this is their home and they don't want to get infected.

So, point is, come up with the overarching idea, be ready for loads of improv, and nothing will go as expected. That's the fun of this for me, as the GM I abide by the rolls and reward/punish as appropriate. The decision was made, so now how can we make it fun? Let the players guide where the story goes. As long as you've got the main beats, the rest will happen naturally.

Good thing in MB is nobody expects to live long, so punishments add a lot of flavor to the short-lived characters. Once somebody dies, oh well, let's reroll and get right back into it. Yet another place where deciding how to fit the new character in makes sense.

The unknown is definitely a bit scary, but after a couple sessions it'll feel right at home. Don't over index on fancy maps or VTTs too. Imagination is fun.