Perhaps Percy is trapped in a time - loop within Camp Half - Blood. Every day he wakes up and the same events repeat, but he gradually starts to notice small differences and figures out that a magical artifact is causing it. He then has to search for the artifact to break the loop.
One plot idea could be Percy having to team up with Hera to rescue a group of demigods who have been captured by an ancient monster. Hera, despite her usual aloofness, realizes she needs Percy's unique skills.
One idea could be that Percy and Hermione meet during a magical event that crosses over the Greek mythology world and the wizarding world of Harry Potter. They team up to solve a mystery that involves both magic and ancient Greek curses.
One idea could be Percy having to team up with an Amazonian princess to rescue a group of half - bloods kidnapped by a new monster. They could face challenges like navigating through the Amazon jungle filled with magical traps.
One idea could be a story where Percy has to team up with a monster that has been wrongly accused. Maybe a centaur is framed for stealing a powerful artifact from Camp Half - Blood. Percy, being the hero he is, decides to help the centaur clear its name and in the process discovers a new threat to the demigod world that involves an ancient monster alliance.
How about a crossover with the Star Wars universe? Percy could be on a strange planet that has a connection to water in a way similar to the Force. He might meet Jedi and Sith and get caught in their power struggles. His ability to control water could be seen as a strange new 'power' in the Star Wars world. Another interesting plot would be a crossover with the Percy Jackson universe and the His Dark Materials series. The daemons could interact with the Greek gods' animal forms, and Percy would have to deal with the complex moral and political issues in both worlds.
Maybe Katara gets transported to Camp Half - Blood. She initially struggles to fit in but then she realizes she can use her water - bending skills in new ways with the help of Percy's understanding of water - related powers. They could then go on a quest together to find a way to send Katara back home, all while forming a strong friendship and learning from each other's cultures.
One idea could be Percy using his gaming skills to outwit monsters in the real world of the Percy Jackson universe. For example, he might use strategies from strategy games to plan attacks against a horde of mythical beasts. Another could be Percy getting trapped in a virtual world that has elements of both the gaming world and the Percy Jackson mythos, and he has to find his way out by solving puzzles based on Greek mythology.
One idea could be Percy seeking help from Hecate to break a powerful curse that has been placed on Camp Half - Blood. Another could be Hecate tasking Percy with a quest to retrieve a magical artifact that she needs to maintain the balance of magic in the world. Maybe Percy has to enter the realm of Hecate and face various magical trials to gain a new power.
One interesting plot could be Percy having to relive his adventures but with a different set of allies. Maybe some of the minor characters from the original series become major players this time.
One plot idea could be Percy seeking Hestia's guidance during a time of great turmoil in the demigod world. Hestia, being the goddess of the hearth, could offer him a sense of home and stability in a chaotic situation. Another idea is a story where Hestia tasks Percy with protecting an ancient relic related to the hearth that has the power to either save or destroy the camp. Or perhaps a fanfiction where Percy discovers a long - lost connection between his family and Hestia, which gives him new powers or a special mission.