One possible plot could be that Harry is wrongly accused of a serious crime within the wizarding world and is sent to Azkaban. There, he discovers a dark secret about the prison itself, like there being a hidden force controlling the Dementors. He has to use his skills and the loyalty of his friends to expose this secret and clear his name.
It could be a corrupted prison guard. They might be taking advantage of the chaos in Azkaban to carry out their own evil plans, like stealing magic from the prisoners. Harry would have to stop them while also dealing with his own situation in the prison.
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.
Well, it could be many reasons. Maybe there was a misunderstanding. Some dark magic might have been used to make his friends think he did something bad. Or perhaps a powerful enemy framed him by planting false evidence. It's hard to say exactly without more context from the fanfiction.
One possible plot could be that Hermione is wrongly accused of a serious crime against the wizarding world, perhaps a dark magic related offense that she didn't commit. As a result, she is sent to Azkaban. Inside Azkaban, she has to deal with the Dementors and the harsh prison environment. Meanwhile, her friends Harry and Ron believe in her innocence and start to investigate to find the real culprit and clear her name.
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.
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.
One possible plot could be that Harry is framed for a serious crime he didn't commit. Maybe someone has used Polyjuice Potion to look like him while doing dark magic. The wizarding community, believing the false evidence, sends him to Azkaban. In Azkaban, he has to endure the Dementors and try to prove his innocence from within the prison walls.
In the fanfic, there might be changes in the security measures of Azkaban. For example, new spells or wards could be added to prevent prisoners from escaping more effectively.
A group of Death Eater sympathizers within the Ministry could be the main antagonists. They still hold onto the old pure - blood supremacist ideals and see Hermione, a Muggle - born witch, as a threat. So they fabricate evidence against her and send her to Azkaban to send a message to all Muggle - borns that they are not welcome in the new wizarding order they are trying to create.