One idea could be that the adopted children have special powers related to different elements, like one can control water in a unique way different from Percy. They could go on a quest to find a lost artifact that is key to their new family's safety.
Maybe the adopted children are from a different mythological background. For example, they could be descendants of Norse gods. Their presence in the Percy Jackson world causes some confusion and also new alliances. They might have to face challenges from both the Greek and Norse sides as they try to fit in and prove their worth.
One idea could be that Percy's niece discovers she has a unique power that no one else in the family has. Maybe she can communicate with sea creatures in a different way than Percy. Another could be that she gets involved in a quest that was thought to be only for the main heroes in the Percy Jackson world, and she has to prove herself to the other demigods.
You can start by giving them unique personalities. For example, one could be really shy but have a hidden strength. Then, put them in different situations to show how they grow. Like when they face a monster, the shy one might find the courage to fight.
Perhaps Percy's niece is suddenly transported to a different realm in the world of Greek mythology. This realm is in chaos due to some dark magic. She has to use her wits and any latent abilities she has to survive and find a way back. Meanwhile, Percy is on a quest of his own but starts to notice strange things that lead him to suspect his niece is in trouble. Eventually, their paths cross and they team up to save the day in this strange new realm.
One idea could be Percy having to deal with the stigma of being associated with Hades in the demigod world. He might face distrust from other demigods who are scared of Hades' reputation.
Well, a plot could involve Percy being sent on a special mission by the Fates. Since they control destiny, this mission might be crucial for the balance of the entire demigod world. He could have to face enemies that are normally out of reach for regular demigods. Along the way, he discovers secrets about his adoption that change his perspective on his life and his role among his peers.
One idea could be Percy having to learn the ways of Artemis's hunters. He might struggle with the no - male - contact rule at first but then find unique ways to bond with his new sisters. Another could be Percy going on a quest specifically related to Artemis's domain, like protecting a sacred forest. And he could use his water powers in new and unexpected ways in the context of Artemis - related tasks.
One idea could be Percy struggling with the dark influence of Kronos while still trying to maintain his loyalty to his friends at Camp Half - Blood. Another could be Percy having to hide his true identity as Kronos' adopted son from his closest allies. Maybe he discovers some long - lost powers or knowledge that Kronos had hidden away for him, which he must use to either save or destroy the world of the gods.
One idea could be Percy exploring his new powers related to Demeter. Maybe he can control plant growth in unique ways, like making plants form protective barriers. Another could be his interactions with Demeter's other children. He might face some jealousy from them at first but then prove himself and form strong bonds.
One idea could be Percy's struggle with his new identity as Nyx's adopted son. He might face discrimination from some of the other demigods who are suspicious of his new connection. Another plot could involve Percy learning new powers from Nyx that set him apart in battles against monsters. He could also have to deal with Nyx's enemies who target him because of his relation to her.
One idea could be Percy struggling to fit in with Athena's children at first. Since he's used to being a son of Poseidon, he has different ways of thinking and acting. Another plot could involve him using his newfound Athena - given wisdom to solve a really complex mystery in the demigod world. Maybe he discovers a new prophecy related to Athena that only he can understand because of his unique position as her adopted son.