Perhaps the Daughter of Gaea is sent on a mission by Gaea to infiltrate Camp Half - Blood. But instead of causing harm, she falls in love with the sense of community there. She starts to secretly help the campers prepare for Gaea's attacks. She could use her connection to the earth, which is different from the demigods' but still useful, like sensing when Gaea is about to send her minions.
One idea could be Percy having to face challenges in a world where Gaea has more direct control over the elements. For example, the earth itself could be constantly shifting and changing, creating new terrains and obstacles for him to overcome. Another could be Percy forming an unlikely alliance with some of Gaea's other children who are also having second thoughts about their mother's destructive plans.
One plot idea could be Gaea's redemption. In the original series, she was a major villain, but in a fanfic, she could be given a chance at redemption. Maybe Percy and his friends discover that Gaea was being controlled by a more powerful and malevolent force all along, and they have to work with her to defeat this greater evil.
One idea could be Percy having to choose between the powers of Kronos and Gaea. Another could be Percy teaming up with some unexpected allies from the realms of Kronos and Gaea to fight a new common enemy. Maybe Percy discovers a hidden prophecy that involves his lineage as the son of Kronos and Gaea and has to fulfill it to save the world.
One idea could be that Percy has a dream where Gaea is trying to corrupt the power of a magical lemon tree. The lemon tree has the ability to heal demigods, but Gaea wants to use it to create chaos. Percy, with his friends, has to protect the tree. Maybe the lemons from the tree can also be used as weapons against Gaea's minions, like when thrown, they release a burst of purifying energy.
Well, a possible plot could be Percy being sent on a quest by Gaea. But instead of following her orders blindly, he uses the quest to gather information about her weaknesses. He could form an unlikely alliance with some minor gods who also oppose Gaea. As for his relationship with his old friends, they would be in shock and some might even try to 'rescue' him from Gaea's influence, leading to some tense and emotional encounters.
One idea could be Percy having strange, prophetic dreams about Gaea that lead him on a quest to understand her true nature before she fully awakens. Maybe he discovers an ancient bond between them that was long forgotten. Another could be Gaea trying to tempt Percy to join her side by offering him power and the ability to reshape the world in a way he might think is better. For example, she could show him a vision of a perfect world without the constant threat of monsters, but with a hidden, dark price.
One idea could be her going on a quest to find a lost trident piece that was stolen by a powerful new enemy. This would involve facing various mythical creatures and using her unique powers as Poseidon's daughter.
One idea could be that the daughter of Hades has the power to communicate with the dead in a unique way, like seeing their memories. She could use this to help Percy solve a mystery that involves an ancient curse. Another plot could be her struggle to fit in among the other demigods because of her dark powers and the stigma associated with Hades. Maybe she discovers a hidden power within herself that is related to her father's realm and has to learn to control it while also dealing with the outside world's view of her.
One idea could be that the daughter of Hecate uses her magic to help Percy in a quest that involves navigating through a magical maze. She could create illusions to deceive the monsters inside.
One idea could be that the daughter of Artemis has unique archery skills that she uses to protect Percy during a dangerous quest. Maybe she can communicate with animals in a special way, like her mother, which helps them gather information or escape from enemies.