Well, perhaps in some fanfiction, there's a plot where the prisoners in Azkaban form an alliance with Harry. They have some knowledge about a dark wizard who is about to rise again and only Harry can stop him with their help. The prisoners might offer him unique magical skills or spells that he can use to combat this new threat.
One interesting plot could be a new adventure where Harry discovers a hidden chamber in Azkaban that holds a powerful magical artifact. This artifact might be related to an ancient wizarding family and its secrets. Another plot could involve the Dementors having a different origin than what is known in the original series, and Harry has to find a way to stop them from spreading a new kind of dark magic.
Some fanfictions might have a plot where Harry goes to Azkaban to rescue someone. It could be a friend or a relative who has been wrongly accused. Along the way, he has to navigate the dangerous corridors of Azkaban filled with various traps and curses. He might also meet some old enemies who are also in Azkaban and they try to stop him from achieving his goal. In addition, the magic that keeps Azkaban intact could be a key part of the story, like Harry has to figure out how to manipulate it to his advantage.
One possible plot could be that female Harry Potter is wrongly accused and sent to Azkaban. There, she discovers a hidden conspiracy within the prison walls. Maybe she finds out that some dark wizards are using Azkaban as a base to plan an attack on Hogwarts. Another plot might involve her using her unique magic skills, different from the male Harry Potter's in some ways, to survive the Dementors and break free, all while uncovering the truth about her false imprisonment.
One interesting plot could be a character from an unexpected house rising to lead all the houses in a time of great peril at Hogwarts. For example, a Hufflepuff might show extraordinary leadership skills during a dark wizard attack.
A possible plot could be a crossover battle. A powerful dark wizard from the Harry Potter world teams up with some sea monsters from Poseidon's domain to attack both the wizarding world and the underwater world. Harry Potter and Poseidon then have to form an alliance to stop this threat. They would have to combine their different powers and strategies. For instance, Harry could use his spells to hold off the dark wizard while Poseidon uses his power over the sea to deal with the sea monsters.
One interesting plot could be Harry Potter exploring his connection to the Peverell family heritage in more depth. Maybe he discovers long - lost magic spells or artifacts related to the Peverells.
One interesting aspect could be how the magic system is tweaked in the fanfic. Since Caer Azkaban is such a dark and foreboding place, the magic used there might have different properties. Characters might have to adapt their magic in order to survive or escape. There could also be a sub - plot about a character trying to reform the prison system in Caer Azkaban, bringing in new ideas and magic to make it less of a hell - hole.
One interesting 'Harry Potter and Azkaban' fanfiction could be about Harry exploring more of the secrets within Azkaban before his third year. Maybe he discovers a hidden passage that leads to an ancient wizarding artifact. Another could be a story where Sirius Black has an entirely different plan while on the run from Azkaban, one that involves helping other wrongly imprisoned wizards escape. And there are fanfictions that focus on the Dementors' origin and how it relates to Azkaban, like a theory that they were created by a dark wizard who once ruled the prison island.
One idea could be exploring what would happen if Sirius Black was actually innocent but had a different, more complex plan than just escaping. For example, he might be trying to uncover a deeper conspiracy within the wizarding world that ties to his imprisonment.
Well, perhaps the child in the 'Harry Potter child in Azkaban' fanfiction forms unexpected alliances. Maybe they team up with some of the inmates who have their own reasons to oppose the unjust system of Azkaban. This could lead to a series of adventures where they try to break free from Azkaban's clutches together.
One interesting plotline could be the exploration of how the vampire element changes the dynamics in Hogwarts. For example, it might create new alliances or rivalries among the students. Maybe the vampire character has unique powers that influence the outcome of certain events, like the Quidditch matches.