In a Jackson's diary fanfiction, it would be interesting if the diary was a record of a future that has not yet occurred. Jackson starts to notice strange entries that seem to predict events before they happen. He then tries to change the future according to what he reads in the diary. Another plot could be that the diary has a curse on it, and every negative thought he writes in it comes to life in a spooky way, so he has to find a way to break the curse.
One idea could be that Jackson discovers a hidden magic in his diary that transports him to a different dimension every time he writes in it. Another could be that the diary holds the secrets of his family's past and he has to solve a mystery to unlock them. Maybe the diary has a sentient entity inside that only he can communicate with and it helps him with his daily life problems.
A really interesting plot could be Percy's diary entries during a time when he is separated from his friends and has to rely on his own wits to survive in a strange new place. He could write about the challenges he faces, the new allies he makes, and his longing to be reunited with his friends.
Another plot could be that Jackson is chasing a group of shadowy villains who are trying to release an ancient evil. He teams up with some unlikely allies during the chase and they all have to use their unique skills to stop the villains.
A great plot could be that the children are sent on a quest to a new, mysterious location that was previously unknown in the Percy Jackson universe. They might have to deal with strange new monsters and make allies among the local mythical beings. For example, they could end up in a hidden valley where the spirits of nature are at war, and they have to figure out how to restore balance. This would involve them using their powers in new and creative ways, just like their parents did in the original series.
One idea could be that the Jackson twins have a secret power that they discover together. They then have to keep it hidden from the world while also trying to figure out how to use it for good.
A plot could be that Sally Jackson plants a lemon tree in her garden that turns out to be a gift from a minor goddess. The lemons from this tree have the power to give those who eat them the ability to see through illusions for a short time. Percy uses these lemons to his advantage when he is on a mission to retrieve a powerful artifact that is guarded by a monster that uses illusions to protect it.
One plot idea could be Percy using his shapeshifting ability to infiltrate the lair of a powerful enemy. He could transform into a small animal like a mouse to get past defenses and then change into a more formidable form to fight. Another could be Percy struggling to control his shapeshifting, accidentally changing forms at inconvenient times, like during a school exam or a tense conversation with his friends.
One idea could be being reborn as a new camper at Camp Half - Blood. You could discover new powers related to water, like being able to create ice sculptures with just a thought, which is a unique take on Percy's water - based powers. Another plot could involve being reborn as a child of a minor god, like Hecate. You'd then have to struggle to prove your worth among the children of the major gods. And perhaps a plot where you're reborn with memories of the future, and you have to use that knowledge to prevent a great disaster at Camp Half - Blood.
A possible plot could involve the diary being used by a group of students who want to learn more about the history of Hogwarts. They start to communicate with the memory of Tom Riddle within the diary, but soon realize that there are dangerous powers at play. As they delve deeper, they must find a way to stop the diary from influencing them too much and causing chaos in the school. For example, they might notice that the diary is starting to make them act more like Tom Riddle in his more malevolent moments.
Another plot could involve the twin sister being cursed. This curse could be related to her connection with the sea as well. For example, every time she uses her powers related to the sea, she experiences a great deal of pain. Percy then has to find a way to break the curse. In the process, they discover an ancient prophecy that not only involves Percy but also depends on his sister's role in fulfilling it.
One idea could be that Percy's little sister discovers she has a unique power that even Percy doesn't have. For example, she can communicate with the spirits of the old gods that have long been forgotten. This could lead to her going on a secret adventure to find out more about these ancient deities and how they can help in the fight against the new threats.