In this kind of fanfiction, there could be various scenarios. Maybe Tails gets into a fight with a really powerful enemy, like a new super - villain. The villain might have some sort of weapon or power that nearly defeats Tails. For example, a weapon that can disrupt his flight or his mechanical abilities.
There could also be the villains. Depending on the context of the fanfiction, it might be Dr. Eggman. He's always trying to cause trouble for Sonic and his friends, and he could be the one behind the situation where Tails almost dies. So Dr. Eggman would be a main character on the opposing side.
Fanfictions about Naruto being neglected and nearly dying can be quite intense. Some might depict him being pushed to the brink by the lack of support from the village. For example, he could be deprived of proper training or basic necessities. However, these situations usually lead to Naruto's character growth in a different way than in the original series. He might form unexpected alliances with outcasts or find new powers within himself.
In some fanfictions, Hermione could be in a situation where she's exploring a dangerous, ancient magical ruin. There could be a series of traps, and she almost dies when she accidentally triggers a deadly one. Her intelligence and quick thinking, along with the help of her friends, might be the only things that save her in the end.
Definitely Naruto. He's always been there for Sakura, no matter what. He would use his Nine - Tails chakra if he had to in order to save her. Sakura's female friends like Ino could also play a role in her rescue. Ino is a skilled ninja in her own right and has a deep friendship with Sakura. And don't forget Tsunade. As the Fifth Hokage and Sakura's mentor in the medical field, she would do everything she could to save Sakura's life.
One interesting Tails Dies fanfiction could be one where his death leads to Sonic's journey of self - discovery and growth. Sonic has to face the reality without his friend and in the process, he becomes more determined to fight against the forces that caused Tails' death.
One scenario could be that he is poisoned by a mysterious potion slipped into his food in the Great Hall. Maybe a Death Eater sympathizer among the students manages to do this.
One common plot could be a battle where Sakura is severely outnumbered. For example, she might be facing a horde of powerful enemies alone while her comrades are occupied elsewhere. Another plot could involve a dangerous experiment gone wrong. Maybe she was trying to develop a new medical jutsu but something in the process backfired and put her life in jeopardy. Also, it could be a situation where she sacrifices herself to protect someone she loves, like Naruto or Sasuke, and almost dies in the process.
The plot might develop by showing how the world changes without Tails. Maybe his inventions were crucial for the protection of their world, and now they have to find new ways to survive. It could also involve other characters trying to bring Tails back through some magical or technological means.
It might be a creative way to rewrite the Sonic story in a more mature or complex way. By having Tails die, new storylines can be created that focus on themes like grief, loss, and redemption. For example, Sonic could struggle with guilt over not being able to save Tails, and this could be the central theme of the fanfiction.