I love the previous answer, but want to add some words of wisdom by my favorite Shakespearean scholar, Harold Bloom.
Bloom argues: "The witchcraft in "Macbeth," though pervasive, cannot alter material events, yet hallucination can and does. The rough magic in "Macbeth" is wholly Shakepeare's; he indulges in his own imagination as never before, seeking to find its moral limits (if any)."
I think you could go a long way pursuing Bloom's avenue of thought. Is he correct? How do the prophecies of the witches impact the outcome of his story? What are the implied "moral limits"? In both cases, you could argue for or against these claims.
Bloom, Harold. "Shakespeare: The Invention of the Human." New York: Riverhead Books, 1998.