In some 'sonic gets killed' fanfictions, it could be due to a self - sacrifice. Sonic might sacrifice himself to save his friends from a deadly threat like a world - destroying weapon. His death then becomes a catalyst for his friends to come together and continue his legacy of protecting the world, while also dealing with the grief of losing 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.
Perhaps in the fanfiction, Sonic gets caught in a trap that was set for him by an evil scientist. The trap is designed in such a way that it slowly drains his energy until he succumbs and dies. His death then causes chaos in the world he once protected, and the other characters have to figure out how to fill the void he left.
One possible plotline could be Sonic struggling to come to terms with his new robotic parts. He might face challenges in his speed or his usual acrobatic abilities and have to relearn how to move and fight. For example, his robotic parts could malfunction at crucial moments.
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.
The world would likely be in chaos. Sonic is often the main defender against villains like Dr. Eggman. Without him, Eggman could easily start taking over cities and enslaving the population. His friends would also be in a state of shock and grief, which could temporarily paralyze their ability to act.
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.
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 is that they meet due to a dimensional rift. Sonic is zapped into Rainbow Dash's world and they have to work together to find a way to send Sonic back. Along the way, they encounter various threats and have to use their combined skills to overcome them.