A great storyline might be Percy as the god of animals trying to protect endangered species in the modern world. He could use his powers to make humans more aware of the importance of these animals. Maybe he creates a magical bond between certain animals and humans who are working towards conservation. This would not only be a great way to blend the modern world with the Percy Jackson universe but also send a strong message about environmental protection.
Perhaps Percy discovers that there is a hidden world of animal spirits within the Underworld. As the god of animals, he feels compelled to explore this realm. Along the way, he meets the spirits of extinct animals and learns about their stories. He then decides to bring some of their essences back to the living world to revive certain extinct species, but this causes a great upheaval as it disrupts the natural order, leading to a series of adventures and battles to fix the chaos he has inadvertently caused.
Some fanfictions might explore what would happen if Percy had a different parentage. Maybe he was the son of a different god or goddess, like Hecate. This would completely change his powers and the way he interacts with the magical world around him. He could learn new types of magic and have a whole new set of challenges and adventures.
One interesting storyline could be Percy as a baby showing early signs of his demigod powers. Maybe he can control water in his crib, like making the bath water swirl around him in a mini - whirlpool. Another could be the reactions of the gods when they discover this new baby demigod. They might be worried about his potential or already planning how to use him in their future schemes.
There could also be a fanfiction where Percy has to face his inner demons in a more psychological way. Maybe he's haunted by the memories of all the battles he's fought and the friends he's lost. His friends then have to help him through this tough time, and in the process, they learn more about each other's deeper fears and insecurities.
One interesting storyline could be where Percy Jackson somehow ends up in Ariel's underwater world. Maybe he's on a quest that takes him there and he has to team up with Ariel to defeat a common enemy, like a sea monster that is threatening both the underwater kingdom and the surface world that Percy is from.
There could also be a fanfiction where the Hunters take in a new recruit who has a mysterious past. This new character might have powers that are unknown to the world of demigods and the Hunters have to figure out how to train and control these powers. Along the way, they discover that this new recruit is somehow connected to an old prophecy that could change the fate of the demigod world.
One interesting storyline could be a Futa Percy Jackson going on a new quest that involves saving a magical realm from an ancient evil. In this fanfiction, Percy's unique form might give her an edge in combat, like being able to use both male and female - like combat styles.
A really engaging storyline could be them being transported to a different era in the world of Greek mythology. They would have to adapt to the different rules and powers that exist in that time while still relying on their own abilities. This would put their friendship and skills to the test in a whole new and exciting way.
One interesting storyline could be a crossover where Percy Jackson meets the Crusaders in a modern-day setting. Maybe they have to team up to fight a common enemy that is threatening both the world of Greek mythology and the historical context of the Crusaders. Another could be Percy using his powers in a way that the Crusaders initially misunderstand, leading to some conflict before they realize they can work together.
One interesting storyline could be Percy's first encounter with a mythological creature as a kid but in a more humorous way. For example, he mistakes a satyr for a really hairy kid at school and tries to befriend it not knowing what it really is.
One interesting storyline could be Percy facing a new and powerful enemy that is a combination of Greek and some other mythologies' creatures. This enemy might be immune to his usual powers, forcing him to find new ways to fight.