You've mentioned several times that the players change what the story is about.....and I get what you're going for, but I don't really think that's an accurate way to phrase it. The players are going to determine how the story goes, absolutely. And I suppose they could change what the story is about if they just decide to walk off and become farmers. But the thrust of the game at that time was the struggle against Strahd. If there's a story, that's what it is. The players can determine if the characters succeed or not, but that's not really the same as changing what the story's about.
I think there is a lot to be said about this.
In
@darkbard and his wife's last Dungeon World session they had made camp below a 400 meter, multi-obstacle wall on their Perilous Journey through/to the top of the mountain to face their ultimate adversary in the game (which became their adversary in session 2 I believe?). This was the 3rd camp of that journey.
Prior to play I had worked up some robust conflict resolution mechanics (a series of Custom Moves and procedures to invest the conflict with tactical and strategic decision-points + harrowing, thematic danger) to resolve the climb. Given the gravity of the effort, this would have likely have taken up a good 1.5 to 2 hours (given the inevitable snowballing complications and resultant game of "Spinning Plates" that would have arisen from the play). Given that our sessions are typically about 2.75 hours, navigating this wall and getting to camp 4 to Make Camp/Manage Provisions and then do End of Session would have likely have been the full content of the session.
However, darkbard's wife (a Wizard) had a spell loaded out that let them transit to the Ethereal Plane and obviate "the wall obstacle" entirely! So the session actually ended up being (a) saying goodbye to their cohorts (a Sherpa and his nephew who was one of their two Porters), (b) a Perilous Journey conflict through the Ethereal Plane to the Feywild barrier doorstep of an enchanted elven village (at a tree line at much higher altitude than should be possible) and, (c) the subsequent showdown with the BBEG with the Wizard's nemesis (an interdimensional, Lovecraftian horror) after a spellcrafting move of hers went south!
Obviously Dungeon World's "play to find out" is the polar opposite of a WotC AP, but what I wanted to focus on was how "the micro story" can change (without affecting the macro story). I had anticipated the session being about climbing a big, scary, multi-obstacle frozen wall! When the reality was, it turned out to be something entirely different (because a Wizard had a move to change the landscape of play...happens even in Dungeon World!)!