The most we can say without spoiling anything is that the last 25% of the adventure is full of some truly epic stuff. Depending on your GM style, you could probably run that section at a higher level. We just don't want to promise it, because in 5th Edition, your mileage may vary, especially if your party is full of min-maxed killing machines. We have enemies ranging up the maximum CR, though, so if you want to throw your party at impossible odds, we've got content spread throughout the world that lets you do that. We also have suggested adventure hooks for continuing the story at higher levels.