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.
Well, in some fanfiction, Sally might die to set up a new power dynamic. For example, if Percy loses his mother, he might be more vulnerable and have to seek new alliances. Also, it could be a way to explore themes of mortality in the Percy Jackson universe. Since Sally is a mortal, her death can highlight the differences between mortals and demigods, and how the demigods deal with the loss of their mortal loved ones.
In fanfiction, Sally's death could be a plot device to drive Percy into a darker place. Maybe the author wants to show how Percy could be corrupted by grief. Or perhaps it's related to a prophecy where her death is necessary for some greater good in the world of the story. It could also be that the writer wants to show the harsh reality of the world Percy lives in, where even the people he loves are not safe, and this could be a way to add more depth and realism to the fanfiction.
In some fanfictions where Sally dies, it could lead to Percy being filled with a new kind of rage and determination. He might become more withdrawn at first, shutting out his friends as he grapples with the loss. But eventually, it could fuel his growth as a hero, making him more fearless in the face of danger because he has already lost the most important person to him.
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.
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.
Perhaps he has lost too many people close to him. If his friends, like Annabeth or Grover, were to die in a particularly tragic way, he could be so grief - stricken that he no longer wants to live in a world without them. He might think that death is the only way to escape the pain of their loss.
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.
Maybe he faced an extremely powerful enemy that he couldn't overcome. It could be a new and terrifying monster created by the Titans or some other dark force.
A possible plot in such a fanfic could be Percy trying to bring Sally back from the dead. He could go on a dangerous and forbidden quest, defying the gods and the natural order. This would put him at odds with many, but his love for his mother is so strong. He might encounter dark magic or make deals with shadowy figures in his desperation, all while his friends are worried about him losing himself completely in this quest.
Perhaps it's a time - travel fanfiction. The lemon is an important item in a different era that Percy and Sally have to retrieve. They have to navigate through different mythologies and challenges to get it, and along the way, they learn more about each other and their own powers.
Another theme is the mystery of Sally's past. Since she has some connection to the demigod world, fanfictions often explore what she might have been through before Percy was born. This could include her own encounters with gods or other demigods, which adds depth to her character.
One possible fanfiction could be about Sally Jackson making lemonade for Percy and his friends during a hot summer day at Camp Half - Blood. It could show a warm mother - son moment between Sally and Percy, with the lemonade as a symbol of comfort and home in the midst of the chaos of the demigod world.