Another storyline could be Percy being called by the Tree of Life itself to become its protector. He would then have to learn about the tree's secrets and how to fend off threats from dark forces that want to destroy it. He might have to train with the spirits that inhabit the tree and gain new powers in the process.
A really cool storyline might be Percy losing his memory and being manipulated by an anti - Olympian force. As he regains his memories, he has to decide whether to continue fighting against the Olympians or rejoin their side. This would create a lot of internal conflict for Percy.
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 Fem Percy using her newfound powers as Poseidon's daughter in a more feminine way. For example, she might use water manipulation to create beautiful ice sculptures during a festival in Camp Half - Blood, which also serves as a form of defense mechanism. Another could be a story where she has to prove herself to Poseidon in a different way than the male Percy did, perhaps by using her charm and diplomacy skills to resolve a conflict between sea creatures that the male Percy would have just fought through. Also, a storyline where Fem Percy discovers a hidden underwater kingdom that was unknown to Poseidon and she has to convince him of its existence and importance.
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.