The story ends where the narrator admits his crime to the police. Perhaps, you could continue the narrative in a jail cell. Here, the narrator/character could obsess over every detail within the cell itself. You could even borrow some of Poe's use of sound and repetition. The "beating heart" might also continue to harass the narrator--but maybe not since that sound was tied to a location. Unlike most of Poe's other short stories, this one is a super (and I mean super) short read. I bet you can read it in less than 30-minutes. Good Luck!