A plot could be that Percy is given a prophecy that involves him having to form a new kind of family unit. He decides that adopting Thalia is part of fulfilling that prophecy. But in order to fully embrace this new family, they need to go on a journey to different mythological realms. In each realm, they face different challenges that test their relationship. For example, in the Underworld, they have to deal with Hades who is not too happy about their new arrangement. This forces them to rely on each other and grow closer as they overcome these obstacles.
One possible plot could be Percy finding Thalia in a difficult situation, like being hunted by some monsters. He decides to take her in and protect her. He teaches her about the demigod world and they form a strong bond as siblings. They train together to become stronger and face various challenges from the gods and other mythical creatures.
Well, one possible plot could be Percy discovering new powers. Since Hecate is the goddess of magic, Percy might learn magical abilities he never knew he had. Maybe he starts to see magic in everyday things in a way he didn't before. For example, he could make inanimate objects come to life or use spells to protect himself and his friends. Another aspect could be his relationship with Hecate. She might be a strict but loving mother figure, teaching him about the deeper mysteries of the demigod world and how magic fits into it all.
Well, one possibility could be that Percy, already a hero in his own right, finds himself in a situation where Apollo decides to adopt him. Maybe Percy has lost his connection to Poseidon for some reason, and Apollo sees his potential. Apollo could then train Percy in new ways of using his powers, not just related to water but also in areas like prophecy or archery, which are Apollo's domains.
One possible plot could be Percy's struggle with his new identity. Since Hera is the goddess of marriage and family, Percy might find himself in a very different family dynamic. He could be thrust into the middle of godly politics more than ever as Hera's adopted son. For example, he might have to deal with the jealousy of other gods' children who see him as having an unfair advantage.
In a 'Percy Jackson adopted by Artemis' fanfiction, perhaps Percy is exiled from Camp Half - Blood due to false accusations. Artemis, who has always been intrigued by Percy's courage, takes him in. With Artemis as his new guardian, Percy gets involved in the Hunters' battles against ancient monsters that threaten the world. He also has to deal with the jealousy of some of the other Hunters who are not happy about a male being in their midst.
One great 'Percy Jackson Adopted Thalia' fanfiction is 'The New Family Bond'. It delves deep into how Percy's decision to adopt Thalia affects their relationship with the other demigods. It has a really interesting take on how Thalia adjusts to her new situation within the camp. Another one is 'Thalia's New Home'. This fanfic focuses on the emotional journey of Thalia as she tries to find her place in Percy's family. It's filled with heartwarming moments and some great character development.
Well, it could be about Percy having to deal with the powers and heritage that come with being the adopted son of Pontus. Maybe he discovers new underwater abilities that are different from his Poseidon - given powers. He might also have to face enemies who are after the secrets Pontus has.
One possible plot could be Percy's struggle with his new identity as the adopted son of Zeus. He might face envy from other demigods who don't understand why he was chosen. He could also have to deal with Zeus' overbearing nature as he tries to train Percy in godly powers. And there would likely be new quests that involve both his ties to Poseidon and his new relation to Zeus.
Well, it could be about Percy having to adapt to his new divine family. Maybe he has to learn special water - related powers from Amphitrite that are different from what he already knows. He could also face challenges from other sea deities who are jealous of his new status.
Perhaps the story could be about Percy facing discrimination from other demigods due to his new relation to Pluto. He is then forced to prove himself not only as a hero but also as a worthy adopted son of Pluto. He trains hard to master the powers and knowledge that come with his new status. He also tries to find a way to use his underworld - related abilities for the good of all demigods, like using his connection to the dead to get information about future threats to the demigod world.