One possible plotline could be that Tails is killed by a powerful enemy during a battle to protect his friends. Maybe a new, extremely strong villain emerges and Tails sacrifices himself to buy Sonic and the others time to escape. Another could be that there is some sort of accident in Tails' workshop while he is working on a dangerous experiment, leading to his untimely death. It could also be that in a dark alternate universe, Tails is targeted and assassinated by an evil organization that wants to disrupt the balance of power in the world he lives in.
In some versions of 'tails gets killed fanfiction', it might be related to a curse. There could be an ancient curse that is awakened in the world, and Tails unfortunately becomes the victim. The curse slowly drains his life force or causes him to be in situations where he can't defend himself. Also, it could be a situation where Tails is trying to solve a mystery that is too dangerous. He gets too close to the truth and the people or forces behind the mystery decide to eliminate him.
One possible plotline could be that Sonic is facing a new and extremely powerful villain who manages to outwit him in a battle and ultimately kill him. This could lead to his friends, like Tails and Knuckles, vowing revenge and setting out on a journey to become stronger to face the villain.
In a 'Tails Turns into a Robot' fanfiction, it could be that an ancient technology is accidentally activated by Tails. This technology fuses with him and turns him into a robot. He then discovers that he has a special connection to an ancient robotic civilization. He has to navigate his new identity, learn about this civilization, and decide whether to embrace his robotic self or try to find a way back to his normal form all while facing challenges from those who fear or want to use his new - found powers.
Maybe Sakura gets the Nine - Tails' power accidentally. This could lead to her struggling to control it at first. As she tries to master it, she could form new relationships with other characters who either help or hinder her progress. For example, some might be jealous of her new power, while others might be intrigued and want to study it.
Another plot could involve Tails trying to hide his sickness at first because he doesn't want to worry his friends. But as it gets worse, he can't hide it anymore. His friends then feel guilty for not noticing earlier and they all come together to take care of him and find a remedy.
One possible motive could be revenge. Another character might have a grudge against Lucy for something she did in the fictional world, like betraying a group or stealing something precious.
One possible plotline could be that it's the result of some strange magic or alien technology. Maybe an enemy uses a device on Goku as part of a plan to weaken him or create chaos. Another could be a more comical take where Goku accidentally ingests something that causes this unusual situation.
Maybe in the fanfiction, Ash is given a Chespin by a mysterious trainer. This Chespin has some special powers that Ash has to figure out how to unlock. As they train together, they form a deep bond and go on to compete in a big Pokémon tournament where Chespin's unique abilities give them an edge.
Another plotline might involve Naruto being ostracized because of his new Mokuton power. Since it was associated with the Uchiha and Senju in a complex way, some in the ninja world might suspect him of having ulterior motives. This would lead to Naruto having to prove himself all over again, this time not just as a jinchuriki but also as a wielder of Mokuton.
A plotline might involve Ash being betrayed by a new acquaintance he meets on his journey. This person could pretend to be his friend, gain his trust, and then use that trust to steal his Pokémon or important items. Or perhaps Ash is betrayed by a Pokémon he just caught. The Pokémon could have been planted by an enemy trainer to spy on Ash and then turn on him at a crucial moment.
In some 'ginny gets drunk fanfiction', the plot might revolve around Ginny being in a new and unfamiliar environment where she gets drunk. This could lead to her exploring a different side of herself. She might meet new people and form new relationships while in this state. The story could then be about how this one drunken night changes her perspective on certain things or people in her life. It could also show her growth as a character as she learns from the experience of getting drunk and the events that follow.