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.
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 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.
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.
Perhaps the characters from the Future Diary world end up in the Naruto universe. Yuno could form an interesting relationship with Naruto. She might see his strong will and be both intrigued and challenged by it. They could team up to fight common enemies, with Yuno's unpredictable and often violent nature contrasting with Naruto's more positive and forgiving attitude.
Well, an OC could be a tech - savvy kid who creates a new app that becomes super popular at school. But there are some glitches that only Greg seems to notice, and they have to work together to fix it. Or perhaps an OC who is a really good skateboarder. He challenges Greg's friends to a skateboarding competition, which causes a rift between the friends at first because some are better than others. Then they all learn to work together to improve. Also, an OC could be a girl who loves reading and starts a book club. Greg's mom forces him to join, and he tries to find ways to make it more fun by adding his own wacky rules.
One idea could be Percy and his friends getting trapped in a virtual game world that's a twisted version of their own. They have to use their demigod skills to defeat the digital monsters and find a way out.
Maybe Percy is sent on a quest to Atlantis because there are rumors of a traitor among the Atlanteans who plans to side with the Titans. Percy has to navigate the complex political situation in Atlantis and figure out who the traitor is before it's too late. He also has to deal with the Atlantean magic which is different from what he's used to.
One idea could be that Percy discovers a hidden realm within the world of the Wanderer. This realm has its own set of rules and challenges that he has to navigate. Maybe he meets new demigods there who have been lost for centuries.