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.
In the 'Tony dies in Siberia fanfiction', it might be that Tony is betrayed while in Siberia. He could be lured there under false pretenses by someone he thought was an ally. Once there, they set a trap for him. Without the support he expected, he's overpowered and meets his end. It could also be related to some experimental technology in Siberia that backfires on him.
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.
It could be a local Siberian warlord who sees Tony as a threat to his power. Maybe Tony's presence in Siberia disrupts the warlord's plans for the region, so he decides to eliminate him.
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.
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.
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.
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.
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.
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.
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 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.