There is no particular mystery in “The Cask of Amontillado,” but it might be asserted that the story is a prelude to one of the greatest mysteries to arise in Venice in many years. That mystery surrounds the question “What happened to Fortunato?” Montresor must realize that there will be an extensive inquiry about Fortunato’s disappearance because he is such an important and influential citizen. Everyone will be talking about this mystery. How could a man just vanish like that? Did he run away for some unknown reason? Was there a woman involved. How were his finances? If he was abducted, why hasn’t a ransom note been sent to his home? If he was murdered, why hasn’t his body been discovered?
Montresor has taken pains to avoid suspicion by pretending to be one of Fortunato’s best friends. Montresor is so accustomed to referring to him as his friend and his good friend that he continues to refer to him in those terms even when he is leading him to his death. When people begin looking for Fortunato and the police are brought in, Montresor will have to display a keen interest in every aspect of the investigation in order to keep up the pretense of being Fortunato’s very good friend. In fact, Montresor might begin his own inquiries, making himself conspicuous in the search for his poor friend. In other words, Montresor might end up investigating his own crime.
If my suggestion interests you, I think the thesis statement would be fairly easy to create. Since Montresor is writing about an event that occurred fifty years earlier, he himself could write about what actually happened when Fortunato turned up missing and how he was involved in the subsequent inquiries and speculations.