A great plot could involve Kairi finding an unexpected ally within the prison of darkness. This ally could be someone who was once an enemy but now has a common goal with her. She could also start to understand the true nature of the darkness and find a way to use it to her advantage, rather than being completely overpowered by it.
Well, you could have a plot where Kairi, while a prisoner of darkness, starts to have visions of the past and future. These visions give her clues on how to escape and also how to prevent future disasters. She might also encounter a series of tests or trials within the prison that she has to overcome using her wits and the latent strength she didn't know she had.
One idea could be that Namine, with her unique power over memories, starts to have visions of a new form of Darkness that is targeting Sora. Kairi, being strong - willed, decides to train harder to protect Sora. Sora, on the other hand, is initially oblivious to this new threat but his bond with his friends starts to give him subtle hints. They then embark on a journey together to face this Darkness, with Namine using her powers to uncover the origin of it while Kairi and Sora fight on the front lines.
Maleficent could be a great villain in this fanfiction. Her connection to dark magic makes her a natural fit for the setting of the prison of darkness. She could be the one who put Kairi there or be trying to manipulate her for her own evil plans.
One plot idea could be that Aqua and Kairi meet in a dream world. They realize they have a lot in common, like their strong wills. They team up to face a new threat that is emerging in the dream world, a dark entity that is trying to invade the real world through the dreams of the people. They use their unique powers, Aqua's keyblade skills and Kairi's latent magic, to fight it off.
One idea could be exploring the backstory of a minor character in 'into darkness'. For example, a crew member who has a hidden past related to the main antagonist. Another could be a time - travel plot where a character goes back in time to prevent the events that led to the 'darkness' in the story. Maybe also a story about a character who discovers a new power or ability that could change the course of the events in the 'into darkness' universe.
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 how Kairi's friends react to the news. For example, Sora might be initially shocked but then become extremely protective. Another plot could revolve around Kairi preparing for motherhood while still dealing with the challenges in her world. Maybe she has to find a safe place to raise her child away from the ongoing battles.
One idea could be exploring the backstory of the 'Prisoner of Sand' character in more depth. Maybe they have a hidden past related to a long - lost shinobi clan in the Sand Village. Another idea is a rescue mission where Naruto and his friends try to free the 'Prisoner of Sand' from some powerful enemy who has captured them for their unique abilities. A third idea could be a story where the 'Prisoner of Sand' has to team up with Naruto to stop a greater threat to both the Leaf and Sand Villages, and in the process, they learn to overcome their differences.
One idea could be exploring what would happen if Sirius Black was recaptured before Harry could prove his innocence. Another could be a story where Peter Pettigrew managed to frame someone else entirely. Or perhaps a fanfic where Hermione uses her time - turning device to go back and prevent Sirius from being sent to Azkaban in the first place.
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.
A plot could involve the Prince of Darkness in a battle against other supernatural forces. He has to team up with unexpected allies, like a group of young witches who are also trying to protect their world from a greater evil. This would lead to a lot of character development as they learn to work together.