Well, one reason is that it allows fans to imagine alternative universes. By killing off Buffy, they can create scenarios where the rules of the Buffyverse are changed. For example, how the balance between good and evil shifts. Also, it can be a way to explore darker themes and emotions. Grief, despair, and the struggle to move on are all things that can be delved into deeply in these fanfics. Moreover, it gives a chance to develop secondary characters more fully as they step into new roles in Buffy's absence.
One popular 'Buffy Dies' fanfic is 'The Aftermath'. It explores how the Scooby Gang copes with her loss in a very emotional way. Another could be 'A World Without Buffy' which delves into the power vacuum her death creates in Sunnydale. There's also 'Remembering Buffy' that focuses on the memories of the characters and how they are haunted by her absence.
Percy might become more independent. Without his mom's presence, he has to rely more on himself. He could start to take on more leadership roles within the demigod community. He also may develop a darker side to his personality, as the pain of his mom's death lingers. However, through his experiences and the support of his friends, he gradually learns to balance this new, harsher side with his original kind - hearted nature.
There could be a fanfiction titled 'Percy's Struggle After Losing His Mom'. In this, it could show Percy having flashbacks of his mom during battles or difficult times, and how those memories both haunt him and give him the will to keep going. It might also include how he tries to honor his mom's memory in different ways.
Another theme is growth. After his mom's death, Percy often has to grow up faster and become more independent. Fanfics might show him taking on more responsibilities and maturing in ways he wouldn't have if his mom was still alive. For example, he may become more decisive in battles or in dealing with other challenges. In some fanfics, there's also the theme of legacy. Percy might discover things about his mom that she left behind for him, like a hidden power or a special message. This gives him something to hold on to and also adds to the idea that his mom's influence still lives on even though she's gone.
One possible plotline could be Percy's journey of grief and how he copes with the loss. He might become more withdrawn at first, then gradually find strength in his friends and his quest. Another could be his search for a way to bring his mom back, perhaps delving into dangerous magic or making deals with powerful beings. Maybe he also has to face the fact that his mom's death was part of a larger plan by the villains and he has to stop them from achieving their ultimate goal.
In the fanfiction, perhaps Artemis decides to take on a more solo approach to her vigilantism. She may feel that with her mom gone, she can't rely on anyone else in the same way. This could lead to conflict within the Young Justice team as they try to bring her back into the fold. Also, she might start investigating the circumstances around her mom's death more closely, uncovering secrets that put her in danger.
Percy often becomes more somber. He may lose some of his initial naivete. For instance, he might be more cautious in his actions, thinking twice before jumping into a fight like he used to.