One scenario could be Lupin sacrificing himself during a major battle against the Death Eaters. He might use his werewolf form to his advantage, taking on multiple enemies at once but getting fatally injured in the process. His last moments could be spent looking at Harry, seeing the hope for a better future in his eyes.
Perhaps Lupin is caught in a dark magic trap set by Voldemort's followers. As he struggles to free himself, the magic drains his life force. His friends try to save him, but it's too late. His death leaves a void in Harry's life, as Lupin was one of the few father - figure like people he had, and Harry would have to grapple with yet another loss.
One scenario could be that she sacrifices herself to save the world from a powerful alien threat. She might make a last - stand alone, knowing that she has no chance of survival but doing it anyway to protect the Earth and her friends. Another could be that she is betrayed by someone she trusts deeply within the Avengers, and this betrayal leads to her death.
One possible scenario could be when Lupin has to leave Hogwarts due to his werewolf secret being revealed more widely. He might cry because he has found a sense of belonging there, with his friends and students. He loves teaching and seeing the young wizards grow, and leaving all that behind is heart - breaking for him.
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.
One scenario could be Gon sacrificing himself to save Killua from a powerful enemy. He might jump in front of a deadly attack, knowing full well it would cost him his life. His last words to Killua could be about their friendship and how he's glad they had their adventures together.
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.
Hermione would be heartbroken as well. She would likely try to find a way to bring Lupin back, delving deep into the library's restricted section for any possible spells or solutions. She would also be the one to comfort Harry and Ron, using her logic to try to make sense of the senseless loss.
One possible scenario could be Deidara getting seriously injured during a battle with a powerful enemy. Maybe he underestimates his opponent and ends up with life - threatening wounds. His comrades might be too far away to help immediately, leaving him to struggle alone.
One scenario could be Hiccup getting seriously injured during a dragon fight. Maybe he's protecting Toothless and takes a direct hit from another dragon's attack. This could lead to a story of his slow recovery, with his friends and family rallying around him.
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.
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.