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.
Maybe in the fanfiction, Luffy contracts a deadly and incurable disease while exploring a new and dangerous island. Despite his will to live and his crew's efforts to find a cure, he succumbs to the illness. This would be a very sad and different take on his character's journey.
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 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.
Perhaps in the fanfiction, Naruto is betrayed during the battle. By someone he trusted, like a comrade who was secretly working for the enemy. This betrayal catches him off - guard, and he is severely injured. Unable to fully recover, he succumbs to his wounds during the fight.
One possible scenario could be that Tony sacrifices himself to save a teammate during a dangerous mission. Maybe he takes on a bomb - defusing task and runs out of time, dying heroically.
One possible scenario could be that Percy is overpowered by the various monsters in Tartarus. Maybe the sheer number of foes like the giants or the more terrifying creatures there are too much for him to handle, leading to his death.
One possible scenario could be that Edward sacrifices himself to save Bella from a powerful new vampire threat. Another might be that he succumbs to an illness that vampires in this fanfic world are suddenly vulnerable to. Or perhaps he is killed in a battle against a group of werewolves gone rogue.
In some 'rwby neo dies fanfiction', Neo might die due to the consequences of her own actions. She could have made a deal with a dark force in order to gain power, but in the end, the power overwhelms her and she succumbs to it, taking her life.