In some fanfictions, there could be a betrayal within Hogwarts that leads to Harry's death in the Chamber. For example, a fellow student who is under the influence of Voldemort or has some dark secret might lead Harry into a trap in the Chamber, and he ends up being killed by the various threats there, like the Basilisk or the ancient magic that lurks.
One possible scenario could be that Harry is overpowered by the Basilisk. Maybe he goes alone to face it without the help of Fawkes or the Sorting Hat's sword, and the Basilisk's venom is too much for him to survive.
In some versions, it could be that Harry has to die to break a dark curse placed on Ginny. He discovers that his life is the key to saving her and willingly gives it up. He might have a premonition about Ginny's fate and decides that his death is the only way to change it. He then confronts the source of the curse and allows it to take his life instead of Ginny's.
One possible scenario could be that Harry loses the love of his life. Maybe she was killed in a battle against dark forces, and he is unable to move on from that grief. His broken heart then leads to his own death as he slowly loses the will to live, consumed by his sorrow and memories of her.
Perhaps in the fanfiction, there's a curse that was meant for Snape but Harry jumps in to take it. As Harry is dying in Snape's arms, Snape has a moment of deep reflection. He might think about all the times he could have been kinder to Harry, and how Harry has now become a symbol of the good that Snape had long forgotten in his own life. And this moment of Harry's death could be a turning point for Snape's character development in the fanfiction.
In such a fanfiction, Harry could die from an accident. For example, if he discovers magic early and while experimenting without proper guidance, something goes horribly wrong. Maybe he stumbles upon a dangerous magical artifact near his home that has a curse and it kills him before he ever gets to Hogwarts. It would be a very different story from the original, as his death would change the whole dynamic of the wizarding world that we know from the books.
Another scenario could be that Gohan contracts a deadly and incurable disease in the fanfiction world. Despite the efforts of his family and friends, including the use of any available advanced technology or magic, he succumbs to the illness, leaving everyone in mourning.
One possible scenario could be that Luffy sacrifices himself to save his crew during a major battle against an extremely powerful enemy. He might use all his strength and energy to protect them, and in the end, it's too much for his body to handle.
The Basilisk is an obvious main antagonist. It is a huge and powerful magical creature that has the ability to kill with a single glance. In the original story, Harry barely manages to defeat it. In a fanfiction where Harry dies, it could be that the Basilisk is just too strong for him to overcome.
One possible reason could be a powerful and unforeseen dark magic spell within the Chamber. Maybe in the fanfiction, the basilisk had some sort of enhanced venom or an additional power that was able to kill Harry despite his usual heroics.
One possible scenario could be that Edward sacrifices himself to defeat a powerful and final enemy. He might use up all his alchemy knowledge and power in one last, desperate attempt to save the world, and his body just can't take the strain.
One possible scenario could be that they sacrifice themselves to save the village from an ultimate evil. They might combine their powers in a final, desperate move that exhausts them completely, leading to their deaths.