An apt essay topic is the main character Willie Loman's deterioration over the course of the play. At the start of the play, Willie is certain of his importance not only in his office and career, but in the lives of his sons, Biff and Happy. Once it becomes clear that he is not in touch with the reality of his work performance or of his sons' success, he rapidly falls apart and eventually ends up killing himself. This awareness of his reality ultimately corrupts him in the most damaging way possible.
Willie's son, Biff, becomes corrupted through the knowledge of his father's affair, which is revealed late in the play. He becomes disengaged from the family and unmotivated to fully pursue a job to support himself adequately. As his father continues his downward spiral, Biff, too, continues to fall apart, eventually pledging at the end of the play to leave his family and never see them again before Willie's suicide.
The American Dream fundamentally means that all Americans have access to true success through hard work--a vision that may not truly translate to reality. Through the Loman family's general deterioration, it becomes clear that the American Dream is not attainable by all, and it is an unrealistic way to view the world.