He could meet some of the older campers who usually don't interact with the new arrivals right away. They might give him some special advice or warnings about the camp.
Percy might also encounter some of the camp's staff members in a more personal way. Since he's early, they could have more time to talk to him and share stories about the camp's past. This could give Percy a deeper understanding of the camp's traditions and his place in it.
There's a chance he meets a new camper who has some unique abilities. This new character could become Percy's friend or rival. For instance, if the new camper has a power related to water like Percy, they might compete to see who is better at controlling it. Or they could team up to face a common threat that shows up early at the camp.
Well, in this fanfiction, Percy might meet some new characters early at the camp. Maybe he discovers a hidden part of the camp that others don't know about yet.
At first, Percy would be in a state of shock. His development would start with this numbness, but as time passes, it would turn into a seething anger. He would start to see the world in a more cynical way. He might become more solitary, spending long hours training or researching how to get revenge. His loyalty to the camp could be tested as he blames the camp's security or lack thereof for Annabeth's death. However, in the end, he could also grow to be more empathetic towards others who have lost loved ones, as he has now experienced that pain deeply.
The plot could involve Percy arriving early to camp and immediately noticing that something is wrong because Annabeth is missing. As he searches for her, he stumbles upon evidence that she has been in a battle with a powerful new enemy. When he finally finds her lifeless body, it shatters him. He then decides to train harder than ever before to avenge her death. He forms a small group of trusted friends from the camp and they set out to track down the enemy responsible, all while Percy is struggling with his grief and guilt for not being there to protect her.
He could meet his old teachers again. They might be surprised to see him back after his long absence.
He might meet alternate versions of his family members. Maybe a Percy from another universe whose mother is a powerful queen instead of a mortal.
There's a chance that Percy meets the personification of Tartarus itself. It could be a shadowy, all - powerful being that tries to break Percy's spirit. Percy, being the brave hero he is, would have to resist and find a way to either outwit or overpower this entity in order to continue his journey in Tartarus.
He might meet his former classmates who have changed a lot since he left. Maybe some new students who are intrigued by his reputation.
Perhaps Percy will meet a group of environmentalists. Given San Francisco's reputation for being environmentally conscious, they could be fighting against some sort of sea - related pollution that Percy gets involved in. He might use his powers and knowledge to assist them, and in the process, make some good friends.
He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.