The plot could start with Harry Potter having a strange dream about Azkaban. He then starts to investigate and finds out that there is a hidden prophecy that says he will own the prison. Once he takes over, he realizes that the Dementors are not as simple as they seem. He has to find a way to control them without using the cruel methods of the past, and also deal with escaped prisoners who are out for revenge on him for changing their 'home'.
One possible plot could be that after the war, the Ministry of Magic is in shambles. Harry Potter, being a hero, is given Azkaban as a form of trust. He then discovers that there are dark forces still lurking within the prison, and he has to fight them off while also trying to reform the prison system.
There might be. Fanfiction authors are known for their wild and imaginative ideas. If Harry Potter were to own Azkaban, it would completely change the power dynamics in the wizarding world. He could use his influence to make the prison a more humane place, or perhaps he discovers some ancient magic bound to the place that gives him ownership. It would also likely cause a stir among the other wizards and witches, both for and against his newfound power over the prison.
Perhaps in this fanfiction, Harry inherits Azkaban due to some dark family secret. He then has to deal with the dark forces that have been lurking within the prison walls for centuries. He might find a way to use the prison's own dark magic against Voldemort's remaining followers who are hiding in the shadows.
In this fanfiction, perhaps Harry goes to Azkaban to rescue someone. It could be a fellow wizard who has been unjustly imprisoned. He might have to use his skills and some new-found magic to navigate the dangerous and dreary corridors of Azkaban, all while avoiding the Dementors.
She could be framed by a dark wizard who has infiltrated the Ministry. Once in Azkaban, she meets an old wizard who imparts some forgotten magic to her. This new magic helps her break free from the Dementors' hold.
Maybe Harry uses magic to create a unique zoo. He could have magical creatures from the wizarding world, like hippogriffs and unicorns. The zoo would be a place for both wizards and muggles to learn about these amazing beings.
One possibility is that Dumbledore discovers new evidence that clears Harry's name. Maybe there was a misused spell by the Ministry that made it seem like Harry was guilty. Dumbledore then uses his influence and magic to break Harry out of Azkaban and they begin to expose the real culprits behind the false accusations.
The plot could revolve around Bellatrix's memories in Azkaban. Since she was there for a long time, she might have had some extremely vivid and disturbing memories. A fanfiction could explore these memories and how they influenced her actions after she escaped. For example, she might have seen the faces of those she killed in her dreams while in Azkaban, and this could have made her more vengeful.
One possible plot could be that Harry, having a deep love for Hogwarts, decides to take over the school to reform it. He might introduce new courses like advanced defensive magic for all years. He could also focus on improving the living conditions of the house - elves, making Hogwarts a more inclusive and progressive place.
In the fanfiction, it could be that Daphne has some unique magical ability that she uses to protect Harry in Azkaban. They might encounter other inmates who are actually innocent too and together they start a mini - revolution against the unjust system of Azkaban. Harry, with his experience of facing Voldemort, might lead the way in finding a way out of the prison.
One possible plot could be that Harry and Ginny are wrongly accused and sent to Azkaban. They then have to find a way to prove their innocence while dealing with the dark and dreary environment of the prison. They might discover a conspiracy within Azkaban that is related to the real culprits they are being framed for.