The sad story of Obito and Rin is a key part of the Naruto narrative. Obito was a young ninja with big dreams and a huge crush on Rin. When Rin was seemingly killed in front of him, it shattered his world. He thought that the ninja world was full of pain and betrayal. This led him to be influenced by Madara. He became one of the main antagonists, all because of his love for Rin and the unbearable pain of losing her. His journey from a hopeful young ninja to a vengeful villain is a tragic consequence of Rin's death.
Kakashi is known for his laid - back yet highly skilled nature. His influence on Naruto could be in terms of teaching him to be more strategic in battles. Rin, being kind - hearted and having a strong sense of care, would give Naruto the emotional support he lacks. So Naruto might become more balanced in his approach to life and ninja - hood.
The main sad point is Rin's death. It was sudden and cruel, and it completely broke Obito. Another sad part is that Obito's love for Rin was so strong that it drove him to the dark side. He lost his way because of his grief.
In a fanfiction of Kakashi and Rin's love story, it could start with their time in the ninja academy. Rin was always drawn to Kakashi's natural talent and mysterious aura. Kakashi, on the other hand, was initially oblivious to Rin's feelings. One day, Rin got injured during a training session. Kakashi carried her all the way to the medical tent. That was the first time he realized how much she meant to him. As they grew older, their bond deepened, and they found themselves falling in love while on various missions.
In an 'Obito kills Kakashi' fanfiction, it could start with a long - standing grudge. Maybe Obito discovers some new information that he believes Kakashi has been hiding from him all these years. This enrages him so much that he decides to take revenge and kills Kakashi in a fit of rage during a battle.
Some fans would be really shocked and sad. Kakashi is a very popular character, and his death at the hands of Obito would be a huge blow to many. They might feel angry towards the author for creating such a tragic scenario.
In this alternate storyline, it could lead to major changes in their relationship and the overall plot. Maybe Obito's feelings would prevent him from going down the dark path he did originally.
In this fanfiction, perhaps the story could focus on how Kakashi and Rin's adoption of Naruto changes the relationships in the village. Naruto might not be as ostracized as he was in the original story. Also, they could help Naruto deal with the Nine - Tails inside him in a more gentle and effective way compared to what happened in the canon story.
It could be Orochimaru. He might be interested in the unborn child for some dark experiment, thinking that the child of Kakashi and Rin could possess some unique abilities. He could send his minions to spy on them or even try to kidnap Rin.