What would be a good expository essay topic to write about for Dante's The Divine Comedy?
I suggest that you do not try to write about the entire Divine Comedy but focus on a single episode or canto. The part that interests me the most personally is the episode involving Francesca da Rimini. Here is a quote from the Wikipedia online encyclopedia:
In the first volume of The Divine Comedy, Dante and Virgil meet Francesca and her lover Paolo in the second circle of hell, reserved for the lustful. Here, the couple is trapped in an eternal whirlwind, doomed to be forever swept through the air just as they allowed themselves to be swept away by their passions. Dante calls out to the lovers, who are compelled to briefly pause before him, and he speaks with Francesca. She obliquely states a few of the details of her life and her death, and Dante, apparently familiar with her story, correctly identifies her by name. He asks her what led to her and Paolo’s damnation, and Francesca’s story strikes such a chord within Dante that he faints out of pity.
The story of Francesca and Paolo has inspired many artists. The Gustave Dore illustrations which are included in some deluxe editions of the Divine Comedy are marvelous. You should take a look at them if you haven't seen them. There is probably a book of his illustrations in your school library. There is one engraving showing Francesca and her lover with Dante.
The punishment given to adulterous lovers in Dante's work reminds me of Anton Chekhov's story "The Lady with a Pet Dog," a work which is often assigned reading for students in college and sometimes in high school. The two married people who fall in love in Chekhov's story end up suffering a fate similar to that of Francesca and Paolo--only their punishment is psychological.
Peter Tchaikovsky's tone poem "Francesca da Rimini" is a wonderful translation of Dante's story into music. It is a short piece, lasting only about fifteen minutes. You might listen to it on a CD from your library if you're not familiar with it.
I think you should focus on a single episode of Dante's long epic poem and not try to cover the whole thing in a single essay.