He could interact with his old friends at Goode, like Rachel. She has a special connection to the demigod world, and their relationship could be further explored. Also, there might be new teachers or students that Percy meets. Maybe a new student who is actually a demigod in hiding too, and they start to figure out each other's secrets.
One possible plot could be that Percy, in an act of trying to prevent a war among the gods, decides to give Hera the master bolt. He might believe that Hera, as the queen of the gods, could be the most neutral party to hold it. This could lead to a series of events where other gods are either enraged or intrigued by his decision. For example, Zeus might see it as a direct challenge to his authority, while Athena might see it as a strategic move. Percy would then have to deal with the fallout of his actions, facing challenges from various mythological creatures sent by the angry gods and also trying to convince the other demigods that his move was for the greater good.
The child could be an orphan from a far - off land that Percy visited. Maybe Percy found the child alone and starving, and felt responsible for the child's well - being, so he brought the child back.
Well, one possibility could be that Percy has been on some sort of adventure and during that time, he discovers a child in need. He decides to take the child under his wing and brings the child back to his world. Then, he has to deal with the reactions of his friends and family, who are both surprised and a bit worried about this new addition.
In this fanfiction, Percy might face new challenges at Goode. Maybe there are new monsters lurking around, or he could be dealing with old enemies who have a new plan. Also, his relationships with his friends at Goode could develop further, like his friendships might get deeper or he could even find a new love interest.
There could be a fanfiction where Percy returns to his family. He has to deal with the years of estrangement. He might start off a bit cold and formal as we know Percy to be, but as he spends more time with his family, he begins to warm up and reconcile with them, especially with his younger siblings who have missed him.
In addition to Annabeth and his family, he might interact with other demigods he knew before. They could be in awe of his new godlike state. There could also be some characters from the godly realm who are interested in Percy's return. They might want to use his power for their own purposes or they could be there to guide him on how to fully control his newfound abilities.
His old friends like Annabeth could be main characters. She might have been leading the group in Percy's absence and now has to adjust to his return. Grover could also be there, still being Percy's loyal friend and perhaps having some new knowledge about the world of magic and nature.
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.
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.