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.
Perhaps Ginny gets hold of a magical object that was once owned by one of the key characters in the 'Prisoner of Azkaban' story, like Peter Pettigrew. This object reveals some secrets about what really happened back then. Harry and Ginny then have to race against time to use this information to save Sirius and also to prevent a greater evil from being unleashed. Their relationship blossoms as they face these challenges together.
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.
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.
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 major plot twist is that Sirius Black is not the villain everyone thinks he is. He was wrongly accused of betraying Harry's parents. Another is that Peter Pettigrew, who was thought to be dead, is actually alive and was the real traitor.
One common plot idea is exploring Sirius Black's life more in - depth. Maybe a fanfiction could focus on his time in Azkaban before his escape, showing his mental state and how he coped. Another idea is changing the roles. For instance, what if Hermione was the one who was wrongly accused instead of Sirius? It would create a whole new dynamic.
One idea could be exploring Sirius Black's life more in depth during his time on the run. Maybe he had some secret encounters with other magical creatures that he never told Harry about.
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.
One idea could be exploring Sirius Black's time on the run in more detail. Maybe he had some secret encounters with other magical creatures that he didn't tell Harry about. Another could be a story from the perspective of Remus Lupin during his time as a teacher at Hogwarts that year, and how he was constantly worried about his werewolf secret being exposed while also trying to protect Harry. A third idea could be a fanfic that focuses on Hermione's time - turning device, and what would happen if she accidentally sent herself back too far in time and had to find a way to get back to the right time without disrupting the timeline too much.
One idea could be Hermione discovering a hidden spell in the Prisoner of Azkaban events that helps her protect Harry in a more unique way. For example, a spell that can temporarily freeze time around a small area, allowing them to escape from danger more easily.