One possibility is that it could be a story of redemption for Bellatrix. Maybe Harry, in his ever - empathetic nature, tries to understand what led Bellatrix to become so evil while they are both in Azkaban. He might try to reach out to the part of her that was once good, perhaps influenced by the idea that everyone has some good in them.
One possibility is that Bellatrix would try to escape and Harry would be there to stop her. This could lead to intense magical battles within the prison walls. Harry, with his strong sense of justice, would not allow her to break free and continue to cause chaos. Meanwhile, Bellatrix, being the deranged Death Eater she is, would stop at nothing to get out and might even try to use Harry as a hostage or a bargaining chip.
One possible fanfiction could be about Bellatrix's time in Azkaban and how she coped with the Dementors. Maybe she had some hidden magic that she used to keep her sanity in that dark place. It could also explore her thoughts about Voldemort during her imprisonment and how her loyalty to him remained unshaken.
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.
A plot twist could be that the Dementors in Azkaban are not what they seem. Harry and Bellatrix find out that they are being controlled by a greater, more sinister force. This discovery makes them put aside their differences and fight together against this new threat, which is something no one would expect considering their prior relationship.
At first, it would likely be extremely hostile. Bellatrix would view Harry as an enemy to be destroyed, and Harry would see her as a symbol of the evil he has been fighting against. But as time goes on, if they are forced to interact, there could be a begrudging respect that develops. For example, Bellatrix might respect Harry's magical abilities that have allowed him to survive against Voldemort so many times.
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.
In the 'harry locked in azkaban fanfiction', it's possible that Voldemort has a hand in getting Harry locked up in Azkaban. Harry, being the strong - willed wizard he is, tries to communicate with the other inmates to find out if there are any clues related to Voldemort's new plot. He also has to deal with the harsh conditions of Azkaban, like the cold and the constant threat of the Dementors sucking out his soul. But he doesn't lose hope and keeps looking for a way to escape and save the wizarding world from whatever danger Voldemort is planning.
One possible plot could be that Fem Harry grows up in the harsh environment of Azkaban, developing a tough exterior. She might discover unique magical abilities due to her unusual upbringing. For example, she could have an uncanny ability to sense the emotions of dementors, which she uses to her advantage. This could lead to her being a target for both the prison guards who fear her power and those outside who want to use her for their own gain.
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.