Well, Obito's sad story began with his misfortunes in war. He was betrayed by the cruel world he lived in. After Rin's death, he felt so alone and betrayed that he completely changed. He was supposed to be a hero but ended up as an antagonist. His life was full of pain and loss. He lost his faith in the shinobi world and in the bonds he once held dear, like his friendship with Kakashi and his comrades. It's a story of how a good person can be broken by extreme circumstances.
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.
One of the key elements is the sequence of tragic events. First, there was the accident during the mission where Obito was severely injured. Then, Rin's death which was a huge blow. Another important element is his relationship with Kakashi. The misunderstanding between them regarding Rin's death was a major factor. Also, his own sense of being a failure due to his physical state contributed to his downward spiral. All these elements combined to create his very sad story.
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.
Naruto could be one of his allies. Naruto has this knack for seeing the good in people and would probably be willing to give Obito a chance to redeem himself. Also, Sakura, with her skills as a medic - ninja, could be an ally. She might help heal Obito if he was injured during the war and see the potential in him to be a force for good.
There could also be interactions with the new generation of ninjas. They might view Obito as a sort of legend or a cautionary tale. Their fresh perspectives could either inspire Obito to change further or make him feel even more out of place in this new post - war ninja world.
Well, the reborn character could explore Obito's Sharingan powers in new ways. Instead of using them mainly for combat like Obito did in the original story, they could focus on using the Sharingan to understand the emotions of others. For example, they could use it to detect lies and hidden motives, which would be a unique twist on the power.
Besides Obito, Sasuke could be a main character in this fanfiction. Sasuke, with his own journey of redemption and his understanding of power and the ninja world, could have an interesting dynamic with the reincarnated Obito. Also, Sakura might play a role. She could be the one who tries to heal the reincarnated Obito, both physically if he is injured during his new adventures and emotionally from the traumas of his past life.
Often, Obito is attracted to Kushina's strength and kindness. He might try to impress her with his own abilities. Their interactions could involve Obito being a bit clumsy in his attempts at first. Kushina, on the other hand, might find his eagerness endearing and start to open up to him more. They could share stories about their pasts, their dreams, which deepens their bond.