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.
It could be the corrupt prison guards or wardens. They might be in league with dark forces outside Azkaban. They oppose Harry and Ginny because the couple is getting too close to uncovering their illegal activities within the prison, like smuggling dark artifacts or collaborating with Death Eaters.
Their relationship could develop as they study together for exams related to the 'Prisoner of Azkaban' events. For example, they might be researching about Animagi and in the process, discover new things about each other. This shared academic interest can bring them closer.
A possible plot twist is that Ginny has a premonition about the real threat in the 'Prisoner of Azkaban' situation. It's not actually Sirius Black as everyone believes, but a dark wizard who has been using the Sirius situation as a distraction. Ginny then teams up with Harry to expose the truth, and in the process, their bond grows stronger.
Harry might survive in Azkaban because of his connection to his friends, even if they betrayed him. Their memories could give him the strength to hold on. And he may be able to use his magic in unique ways. He could draw on the positive emotions he has left, like his love for his parents, to create a shield against the Dementors. Plus, he may be constantly looking for a way to prove his innocence and that hope could keep him alive.
He might use the invisibility cloak. Harry could sneak into Azkaban unnoticed and then use magic to break Draco out of his cell. The cloak has served him well in the past, and it would be a simple yet effective way to get past the guards.
Maybe he uses some powerful magic spells that he has learned secretly. For example, the Patronus charm could be used to fend off the Dementors which are the main guards in Azkaban. He might also get some unexpected help from his friends or some mysterious characters within the fanfiction world.
It could be Sirius Black. Since he has his own experience with Azkaban, he might know some secrets about the place. He could return in some form within the fanfiction and assist Harry in his escape, maybe by leading him through some hidden tunnels or using some special skills he developed during his time in prison.
Well, in some fanfictions, Harry might use some hidden magic he discovers in Azkaban. Maybe he finds a way to turn the Dementors' power against Voldemort. Since Dementors can suck out a person's soul, Harry could somehow direct them towards Voldemort, weakening him enough to deliver the final blow.
In the fanfiction, the female Harry Potter might be sent to Azkaban as part of a plot by Voldemort to get rid of her. While in Azkaban, she discovers a hidden power within herself. It could be related to her connection with her mother or some ancient magic. She then starts to influence the other inmates with her positive energy, and they start to believe in her as a symbol of hope against Voldemort.