One possibility could be that Percy returns with new powers. Maybe he has some connection to the Underworld now that he's come back from the dead. He might be able to control the dead or has some knowledge about the afterlife that he uses to help his friends in battles against new and more powerful enemies.
Maybe it's a new character who is jealous of Percy's status among the demigods. This new antagonist could be a powerful demigod from a different camp who has heard of Percy's feats and doesn't want him to steal the spotlight again. So, they try to use Percy's return from the dead as a way to turn the other demigods against him.
One possible plot could be that Percy returns with new powers. Maybe he was resurrected by some ancient magic or a powerful deity who has a plan for him. He then has to deal with the shock and awe of his friends and also face new enemies who are either threatened by his return or want to use his newfound powers for their own gain.
It could be a jealous demigod who thought that with Percy dead, they would be the most powerful or important one. Now that Percy is back, they see him as a threat to their status and plan to get rid of him again.
Some of the more powerful and vengeful spirits in Tartarus could be the main antagonists. These spirits could possess demigods or mortals close to Percy to get to him. They might be angry that he escaped their domain and want to drag him back. Additionally, there could be a group of rogue demigods who believe that Percy has been corrupted by Tartarus and they see it as their duty to stop him, even if it means becoming his enemies.
One possible plotline could be Percy having nightmares about his time in Tartarus. He might struggle with PTSD, which could affect his relationships with his friends and family at Camp Half - Blood. Another could be that he brings back some sort of dark power or knowledge from Tartarus that he has to figure out how to control or use for good. Maybe he discovers a way to close the entrance to Tartarus permanently so that no more monsters can come out and endanger the demigod world.
In some fanfictions, Percy Jackson might be depicted as dying in Tartarus. However, this is a departure from the original series. Fanfiction writers often take creative liberties, and some may choose to create such a tragic scenario for shock value or to explore different themes like sacrifice and the finality of death in a harsh, mythological setting.
In the 'Percy Jackson returns to Goode' fanfiction, it could be that Percy has to face some challenges related to his dual identity. On one hand, he is a normal student at Goode, on the other, he is a demigod. This could lead to some interesting situations where he has to balance between the two worlds. For example, he might have to hide his demigod skills during a school competition but still manage to win in a rather unexpected way. Also, there could be new characters introduced, perhaps a new demigod who also attends Goode and Percy has to team up with them to face a common threat.
Well, he might face new monsters. For example, some ancient and more powerful versions of the ones he's already fought. Maybe there are new quests waiting for him at the camp, like finding a lost artifact that has the power to either save or destroy the camp.
A group of rogue demigods could be the main antagonists. They believe that Percy is too powerful and his presence is causing more problems than solutions in the demigod world. So they make him disappear. But when Percy comes back, he shows them that they were wrong and that they have been mislead by false beliefs.