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.
A cool plot twist might be that the time - turner Hermione uses has some unforeseen side effects. Instead of just allowing her to attend more classes, it starts to create glitches in time that cause different events from the past to bleed into the present. For instance, old spells from the founders of Hogwarts start to randomly appear in the corridors, and the characters have to figure out how to stop this chaos while also dealing with the Sirius Black situation.
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.
Perhaps Harry Potter travels back in time and ends up befriending a younger Bellatrix. He could try to influence her to not become a Death Eater. This would create a really complex situation because he has to balance not changing too much of the future while also trying to save her from the dark path. It could lead to some really intense moments where Harry has to hide his true identity and motives from her.
Maybe in the fanfiction, Hermione gets accidentally sent back in time instead of Harry when they are trying to save Sirius. She then has to use her wits to find a way to not only save Sirius but also to get back to her own time, which adds a whole new layer of complexity to the story.
One interesting plot twist could be that Sirius Black is actually being framed by a more powerful dark wizard who has the ability to disguise as others. This would add an extra layer of mystery.
In some fanfics, Lupin might turn out to be under the influence of a spell that makes him seem more dangerous than he actually is. Maybe the real threat is a hidden character who is using Lupin's condition to create chaos. For example, they could be making him lose control of his werewolf form more often than normal, which would lead Harry and his friends to suspect Lupin wrongly.
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.
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.
A fascinating plot twist in certain Harry Bella fanfiction is when their memories are swapped. Harry experiences Bella's life in her world and vice versa. This allows the readers to see how they would react in each other's shoes. It also leads to some really interesting character development as they try to understand each other's worlds better while also trying to find a way to get their memories back to normal.