It may also divide the fandom a bit. Some fans who are more attached to the original positive and heroic narrative of Harry may not like the idea of him dying in fanfic at all. While others, who enjoy exploring darker and more alternative storylines, might embrace these 'harry potter dies fanfic' and even contribute to creating more similar stories, which can cause some friction between different groups of fans.
One possible fanfic could be titled 'The Grief of Harry: When Sirius Fell'. In this fanfic, it might show Harry's immediate reaction of shock and disbelief. He might stand frozen as he witnesses Sirius's death, then slowly sink to his knees, tears streaming down his face as he screams in anguish. His world would shatter, and he'd feel a loneliness like never before.
Well, in this kind of fanfic, Harry could be neglected by Hogwarts staff. For example, teachers may be too preoccupied with other matters, such as political unrest in the wizarding world. Harry, feeling alone and abandoned, might venture into dangerous areas like the Forbidden Forest more often. He could encounter a powerful and malevolent creature that he's not ready to face, and ultimately die because there was no one to rescue him. Also, the Sorting Hat could potentially mis - sort him into a house where he doesn't fit in at all, and the neglect from his housemates could contribute to his downfall.
One possible plotline could be that Harry Potter sacrifices himself to defeat an even more powerful dark wizard than Voldemort. Maybe this new enemy has a power that can only be countered by Harry's self - sacrifice, which would lead to a very bittersweet ending for the wizarding world.
If Harry dies instead of Cedric in fanfic, Hogwarts would be in a state of deep mourning. The entire wizarding world that had placed so much hope in Harry would be in shock. Voldemort would likely face less opposition in the short term as Harry was seen as the main threat to his return. Friends like Ron and Hermione would be devastated, and they might have to take different paths. Ron could become more withdrawn, and Hermione might focus all her energy on finding a way to avenge Harry. The story would also likely shift focus to Cedric, who would now carry the weight of the prophecy or the task of ultimately facing Voldemort, if there was such a concept in this altered universe.
A plotline could be that Ron is killed in a magical accident during an experiment gone wrong. This accident might be related to some new, dangerous magic that he was trying to master. His death would shatter the trio. Harry would feel guilty because he might have been involved in the experiment in some way. Hermione would be grief - stricken and would pour herself into research to try and understand what went wrong. The fanfic could then explore how the wizarding community reacts to Ron's death, with some blaming Harry and Hermione, and others mourning the loss of a great wizard. It could also show how Harry and Hermione have to grow and become stronger individually to face the challenges that come their way after Ron's death.
The wizarding world would be in chaos. They would realize too late that they had pinned all their hopes on Harry and neglected him. There would be a great sense of guilt and despair. The Hogwarts students would be shocked and many would feel a deep sense of loss. The teachers might question their own actions and inactions that led to this. The Order of the Phoenix would be in disarray, having lost their main weapon against Voldemort.
One possibility could be that Voldemort manages to find and kill Harry much earlier. Maybe the protection spell that Lily cast fails due to some unforeseen dark magic interference. This would change the entire wizarding world. Hogwarts might be taken over by Death Eaters completely, and there would be no 'Boy Who Lived' to inspire hope. The Order of the Phoenix would likely be crushed much sooner, and the wizarding world would descend into a much darker and more oppressive state than in the original story.
One great fanfic could be 'The Aftermath of Sirius'. It delves deep into how Harry copes with the loss. It shows his internal struggle, his nightmares, and how he tries to find meaning in Sirius' sacrifice. Another is 'Remembering Sirius' which focuses on the memories of Sirius shared among the Order of the Phoenix members. They recall his bravery, his loyalty, and how much he meant to them all. There's also 'Sirius' Legacy' which looks at how Sirius' death impacts the wizarding world at large, not just Harry.