Perhaps Stiles is killed as a sacrifice in a dark ritual. His death could be part of a much larger conspiracy. His friends might start seeing strange visions of him after his death, which they initially think are just their minds playing tricks on them, but later realize are actually Stiles trying to communicate from the afterlife to help them stop the forces behind his death.
In a fanfiction, Elsa might be killed as a sacrifice. There could be an ancient prophecy that states that in order to save Arendelle from an eternal winter that is a result of an even more ancient curse, Elsa, as the most powerful magic - wielder, has to give up her life. She accepts this fate willingly, and as she dies, her last act is to break the curse, leaving her people in a state of shock and mourning.
One possible plotline could be that Stiles' friends immediately start a search. They might follow clues like strange footprints or a mysterious note left behind. Another could be that Stiles manages to outwit his kidnappers in some way while being held captive, perhaps by tricking them into revealing their weaknesses. Maybe the kidnappers are after something Stiles knows, like a secret about the supernatural world in the fanfiction's setting.
Perhaps the fanfiction is following a specific plot arc that doesn't naturally involve Stiles. Like if it's a story about a new villain in town and the existing characters are dealing with it in a way that doesn't require Stiles' skills or presence. Also, some writers might be influenced by popular trends in the fandom that don't feature Stiles, and they go along with that to attract more readers.
In a fanfiction, it could be that a scientific experiment gone wrong in Gotham creates a mutant - like creature that has super strength and intelligence. This creature targets Batman as it sees him as a threat to its new - found power in the city. After a long and brutal battle, the creature overcomes Batman and kills him.
One possible plotline could be that Harry gets hurt during a Quidditch match. Maybe a Bludger hits him hard and he has to deal with a serious injury like a broken bone. He then has to struggle with not being able to play for a while and the fear of not being as good when he recovers.
Perhaps Mikasa gets sick right before a big battle or important mission. She is determined to still participate, but her body won't allow it. This causes her to feel frustrated and guilty. Her comrades then have to step up in her absence, and she has to watch from the sidelines as they face danger. Meanwhile, she focuses on getting better so she can rejoin them.
One possible plotline could be Belle getting hurt while exploring the forest. Maybe she stumbles upon a hidden trap set by some hunters. Another could be that she gets injured in an accident at the Beast's castle, like falling down a flight of stairs. Or perhaps she gets hurt while trying to protect the Beast from an angry mob.
Another plot could be that Theo kidnaps Lydia as a way to get back at the McCall Pack. Since Lydia is an important member of the pack, taking her would be a major blow. As she's held captive, Lydia discovers Theo's deeper motives which are related to some past event involving her friends. This could lead to a tense stand - off when the pack comes to rescue her.
One possible storyline could be a battle scenario where the enemy is far too powerful for Sakura. Maybe she's outnumbered and despite her best efforts in combat and using her medical ninjutsu to heal herself and allies, she eventually succumbs to the attacks. Another could be a betrayal within her own group, where someone she trusts stabs her in the back, literally. This would add a lot of drama and angst to the story.
One possible plotline could be that it's a magical accident. Maybe a spell gone wrong in Asgard causes this unexpected situation. Thor is initially shocked but then decides to be supportive. Loki, on the other hand, is more conflicted, dealing with his own emotions and how this will change his standing in Asgard.