One scenario could be that he is poisoned by a mysterious potion slipped into his food in the Great Hall. Maybe a Death Eater sympathizer among the students manages to do this.
In some fanfictions, there might be a powerful dark magic spell gone wrong during a Defense Against the Dark Arts class. For example, a spell that was supposed to be a demonstration gets out of control and targets Harry directly. It could be a spell that creates a dark vortex that starts sucking Harry in and he nearly dies before the teacher manages to reverse it.
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.
One common scenario could be in a battle against dark wizards. Maybe he's outnumbered and almost gets hit by a deadly curse. For example, a Death Eater could aim the Avada Kedavra curse at him when he's distracted trying to save a friend.
It could be a new dark wizard who has infiltrated Hogwarts under the guise of a new teacher. This wizard might have a vendetta against Harry specifically and sets up elaborate traps to kill him.
In some scenarios, Robin might be facing a powerful supervillain alone and gets severely injured. For example, he could be up against a foe with enhanced strength and speed, like Deathstroke. The villain could have a well - laid - out trap that Robin stumbles into during a solo mission.
One possible scenario could be in a battle against a powerful dark wizard. Hermione might sacrifice herself to save Harry from a fatal curse. She could jump in front of him just as the curse is about to strike, taking the full brunt of it. This would be a very emotional moment as Harry realizes what she has done and is left to face the aftermath without her.
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 interesting scenario could be them exploring hidden parts of Hogwarts together. They might find a secret room filled with old magic artifacts that have been long forgotten. This could lead to an adventure where they have to figure out how to use or protect these items from falling into the wrong hands.
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.
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.
Maybe they attend a pre - Hogwarts magic class together. Harry struggles a bit with some of the advanced spells being taught, and Daphne, though initially a bit aloof, decides to help him. This could start a relationship where they learn from each other, with Daphne learning about Harry's unique experiences outside the wizarding world and Harry getting a better understanding of the pure - blood society through Daphne.