I assume you are referring to the first proposal, in Volume II, Chapter 11. Because this scene represents the height of misunderstanding between Elizabeth and Darcy, it promises to yield much of thematic importance.
1. One possible topic could involve how both Elizabeth and Darcy demonstrate the qualities of pride and prejudice in this scene; how Elizabeth’s prejudice and Darcy’s pride fuel the hostility of the encounter and the near destruction of their chance for happiness.
Elizabeth and Prejudice: It is important to take note of Elizabeth's state of mind just before Darcy's proposal. Because of Colonel Fitzwilliam’s earlier revelation that Darcy had intervened and effectively ended the courtship between Bingley and Jane, Elizabeth is more disposed than ever to despise Darcy—to be guilty of prejudice. She is also blinded by prejudice because of her ignorance—some might say willful ignorance—of the truth about Wickham’s character. Near the end of the scene she admits that “from the very beginning” she had been convinced of Darcy’s faults and had formed a “ground-work of disapprobation.” Elizabeth’s prejudice for him has prompted her to “willfully misunderstand” Darcy, a charge he correctly makes earlier in the novel.
Darcy and Pride: In this scene, Darcy’s pride is most evident in his repeated remarks about the inferiority of Elizabeth’s family. His confidence that Elizabeth will accept him, despite his insulting remarks, demonstrates a surfeit of both pride and arrogance. Moreover, he is unaware of his own hypocrisy: while accusing Elizabeth of reacting to him out of injured pride, he goes on to boast about his superior place in society.
2. After a close reading, you may discover that neither character is guilty of possessing only one of the flaws alone. How is each character guilty of the same fault with which they earlier denounced the other?
Elizabeth and Pride: Elizabeth’s pride in her own judgment and wisdom has fed her dislike for Darcy throughout the novel. It enables her to carelessly show off her intelligence and to feel justified in aiming her sharp wit at Darcy, often unfairly. Elizabeth’s confidence in her judgment also leads to her unthinking acceptance of Wickham’s story of his treatment by Darcy. In this scene, notice how Elizabeth’s pride is wounded throughout. Although her anger is understandable, observe how it leads to her devastating declaration of rejection near the end. (Only a short time later, she perceives how premature and reckless some of her impressions have been, and she continues to make this discovery throughout the rest of the novel).
Darcy and Prejudice: Darcy’s pride in his superiority of his situation and character is no doubt grounded in his prejudice against people beneath his rank. Up until this point in the novel, we have witnessed him denounce Meryton and its inhabitants on many occasions. He seems unable to acknowledge the value of people below his station (except for his own intimate family servants). In this scene he seems unable even to credit Elizabeth with the sensitivity to be hurt by the more offensive features of his proposal. Not until she makes the accurate observation that Darcy had failed to act in a “gentleman-like manner” does he seem cognizant of any wrongdoing on his part, much less the degree to which his words had provoked her.