It could be his own mother. She might realize that he has become too dangerous and, in a moment of heart - breaking decision, find a way to destroy him. Maybe she discovers a weakness and uses it against him, driven by her love for humanity and the knowledge of the destruction he could cause.
It could be a new and powerful dark side user. This character might have emerged from the shadows, having trained in secret, and their sole purpose is to take down Anakin as he is seen as a threat to their plans for galactic domination.
Well, in a fanfiction, he might be killed by his own hubris. Let's say he continuously underestimates the power of human ingenuity. A group of scientists manage to create a device that disrupts his energy source. While he's weakened, the military launches a full - scale attack using advanced weaponry specifically designed to target his unique physiology. So, his overconfidence in his invincibility leads to his downfall.
Well, Chopper in a really comical and out - of - the - box fanfic could transform into a human girl (through some wacky magic or something) and give Luffy a kiss just for fun. Usopp's sister (if we assume he has one in the fanfic) who has heard all about Luffy's adventures and is a big admirer could be the one kissing him. And then there's always the chance that it could be a random girl from an island they visit. Maybe she's a baker's daughter and Luffy saved her bakery from being burned down by pirates, so she gives him a kiss as a thank you.
It could be a long - lost relative of an old enemy. For example, if there was a powerful dark wizard that Natsu defeated in the past, this relative might come seeking revenge and be powerful enough to kill Natsu.
It could be a power - hungry Pokemon trainer. This trainer might see Gladion as a threat to their goal of dominating the Pokemon league or the entire Pokemon world. So, they plan and execute his killing.
Another possibility could be a new character introduced in the fanfic. This new character might be using Naruto to gain something, like power or influence in the ninja world, and when they get what they want, they cheat on him. It would add an element of mystery and betrayal to the story.
His father, Minato, could be a likely candidate in a really dark fanfiction. Maybe he was resurrected in an imperfect way and his mind was warped, causing him to turn on Naruto.
It could also be a new villain created just for the fanfiction. This villain might be someone who has a personal vendetta against Steve. Maybe Steve foiled their plans in the past, and they finally get their revenge. They could have powers that are specifically designed to counter Steve's abilities.
A new, unknown villain. Maybe a brilliant scientist who turns to the dark side after a personal tragedy. This villain could create a weapon specifically designed to counter Batman's abilities, like a suit that can absorb all of Batman's punches and then redirect the force back at him, ultimately killing him.
It could be Vegeta. Vegeta might kick Goku out of a training session because he feels Goku is showing off or not taking the training seriously enough. Vegeta has always been competitive with Goku and this could be his way of asserting his dominance.