If J. K. Rowling had written Harry Potter and the Goblet of Fire as a tragedy, the protagonist would not be Harry Potter, but Barty Crouch, Sr.
Now, upon investigation, the story of Barty Crouch, Sr., does not follow the Aristotelean definition of tragedy; still, I think there is no question that Crouch is a tragic figure, at least if tragedy is loosely understood.
I want to explore the tragic story of the Crouch family because upon further reading of and reflection upon The Goblet of Fire I believe that it is this story that drives the plot. It is in many ways one of the most important elements of the book, and the degenerate form which it took in the film was a weakness. Barty Crouch, Sr., became a timid old fart; and Barty Crouch, Jr., a cardboard nutjob with a tic. One of the most enjoyable aspects of The Goblet of Fire (the book, I mean) is the discovery of the wider wizarding world, to which Harry has been (despite his importance as the Boy Who Lived) a peripheral figure, and which has a life and energy of its own apart from Harry and Hogwarts. The story of the Crouches is part of that world.
Dumbledore's epitaph on Barty Crouch, Jr. is applicable in part to his father: 'see what that man chose to make of his life! [p, 615]' We shall see how essential the tragedy of Barty Crouch, Sr., is to the plot of The Goblet of Fire, how the Crouch family dynamics and relationships implied or discussed in the book imitate, in however fantastic a way, those of ordinary families, and what lesson, if lesson there is, we can take from the example of the Crouches.