Maybe in the fanfiction, Spider Man is poisoned by a new type of venom created by a mad scientist. Despite his attempts to find an antidote, his condition worsens until he eventually dies, leaving the city in mourning and his friends and family heartbroken.
One possible scenario could be that Spider Man sacrifices himself to save the city from a powerful and unstoppable villain. He might use all his strength and powers to defeat the enemy, but in the process, his body gives out due to the extreme exertion.
In some fanfictions, Peter might die heroically while saving the city from a powerful villain like Venom. He could sacrifice himself to protect his loved ones and the innocent.
Another scenario could be that Iron Man is poisoned or infected with a deadly alien virus. He tries to fight it off but eventually succumbs to it. His friends in the Avengers try their best to save him but are unable to. This would be a really tragic way for him to go, especially considering how much he has done for the team and the world.
Perhaps he succumbs to an old wound or illness that has been haunting him throughout the story. In fanfiction, the author might play with the idea of his age finally catching up to him, and he dies a peaceful death in his sleep or while reminiscing about his life.
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.
He could become more withdrawn. Losing Aunt May, who was like a mother to him, would be a huge blow. He might isolate himself from his friends and even from his crime - fighting duties for a while as he deals with his grief.
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.
One possible scenario could be that Tony is in Siberia on a mission gone wrong. Maybe he's facing a powerful enemy there, and despite his best efforts, he gets fatally injured in a fight. Another could be that the harsh Siberian environment plays a role, like he gets caught in a blizzard and succumbs to the extreme cold.
One possible scenario could be that Percy sacrifices himself to save Annabeth from a powerful and deadly monster. Maybe a huge and ancient sea creature that only Percy can hold off long enough for Annabeth to escape, but at the cost of his own life.