Every single everything would need to be changed to the point of it basically being an entirely new product
You can't have multiple parties running around in a world designed for single-party questing.
Take the battle in front of the grove as just one example. Will multiple parties be able to join the fight? That would be dumb because that fight is already so easy.
So then what? Multiple different fights at once for each party present? And what if Wyll eats it in one party's fight, but not another? And once those fights are resolved, multiple parties in the jail with Saza and the tieflings?
The number of problems that would need to be figured out, it's basically making a whole new game, but harder
