Sally could be in a new relationship or have new friends that are demanding a lot of her time. As a result, she may not have as much time or energy to devote to Percy, which unfortunately comes across as neglect in the fanfiction.
Percy might deal with the neglect by becoming more self - reliant. He could focus on his own interests and skills, like training in combat or studying magic if that's relevant in the fanfiction's setting. This would give him a sense of purpose outside of his relationship with Sally.
Well, Percy is often neglected in fanfiction because he doesn't have the same kind of popularity as the main characters. In the books, he's not part of the core group that goes on the big adventures. He's more into the rules and regulations of the wizarding world. Fanfic writers usually prefer to write about characters who are more involved in the action - like Harry facing Voldemort. Percy's role in the official story is more of a background one in terms of the main plot, so it's easier for him to be overlooked in fan - created stories.
One reason could be that the writer wants to create a more angsty and dramatic story. Sally's death would be a huge shock to Percy and the readers. Another reason might be to test Percy's character, to see how he would handle such a great loss.
Another reason might be that she is forced to leave. There could be a magical or divine force at play. Maybe a god or goddess, who has some grudge against Percy or his father Poseidon, manipulates the situation so that Sally has no choice but to abandon Percy. This would add a lot of drama and conflict to the fanfic.
His parents could be too busy with their own lives. Maybe they have demanding jobs or are preoccupied with their own personal matters, leaving little time to attend to Edward.
Perhaps Sally has some ulterior motive for seducing Percy. Maybe she's trying to get information from him or is part of a plot against him that she later has a change of heart about. She could use her charm and beauty to draw him in. She might start by dressing in a way that catches his attention and then use her wits to keep him interested. Along the way, Percy could be initially oblivious to her intentions but then start to fall for her as well, leading to a complex relationship dynamic.
How about a park? A park where Percy used to play as a child. There are big oak trees and a little pond. Sally could be sitting on a bench feeding the ducks when Percy comes up to her. The peaceful environment of the park would contrast with the adventures Percy has been on, making their reunion even more special.
I'm not sure about the specific plot as it's a rather inappropriate concept. Fanfiction should generally be based on positive and ethical themes.
In a 'Percy and Sally Reunion' fanfiction, Percy could return after being presumed dead. Sally has been mourning him all this time. His sudden appearance shocks and delights her. They hug tightly and don't let go for a long time. Percy then shows Sally his new powers or skills he's acquired during his time away. They also plan for a future where Percy can stay closer to home and help protect it, and Sally is just happy to have her son back in her life.
First, in the original Percy Jackson story, Sally is a loving mother. Changing this fundamental aspect of their relationship goes against the established character traits. Second, it's an extremely disturbing and inappropriate concept, especially considering the target audience of the Percy Jackson series which includes young readers. Third, it violates the positive and family - friendly atmosphere that the series has created.