In a fanfiction, Obito might be sent on a mission by some unknown force to watch over Baby Naruto. As he does, he starts to question his own beliefs. Naruto's pure and unwavering will even as a baby might start to chip away at Obito's hardened heart. This could lead to Obito trying to influence Naruto's growth in a positive way, like teaching him some skills secretly without Naruto knowing it was Obito.
One possible plot could be that Naruto, with his unwavering belief in friendship, manages to reach out to Obito who is still lost in his dark thoughts. Naruto shows Obito the true meaning of the village and how people can change for the better. They could then team up to fight a common enemy, like a new powerful threat to the Hidden Leaf. In the process, Obito starts to heal from his past traumas and realizes that he can still have a positive impact on the world through friendship with Naruto.
One possibility could be Naruto having to deal with Obito's past traumas and memories while trying to use his own ninja skills. He might struggle to convince Obito's old allies that he's not the same person. For example, he could have to face Kakashi and try to explain the situation without revealing too much of Obito's secrets.
One possible plot twist could be that Obito's true intention in founding Naruto's world was not what it seemed. Maybe he was trying to create a utopia where there would be no more war, but his methods were extreme. For example, he might have planned for Naruto to be the ultimate sacrifice to activate a jutsu that would change everyone's nature to be peaceful. Another twist could be that Obito was being controlled by a greater, more mysterious force all along and Naruto was actually a key to break free from that control.
Perhaps in the fanfiction, baby Naruto could accidentally use some of the Nine - Tails' chakra in a positive way that helps the village without anyone realizing it was him. For example, he might unknowingly heal a sick part of the forest near the village, and the villagers start to notice strange but good things happening.
The fanfiction could have a plot where Obito's reincarnation is not a normal one. He is reincarnated with a special mission from the afterlife. He has to find a way to bring peace to the shinobi world in a different way than Naruto did. He might have to face new enemies and also deal with the old grudges that still exist among the ninja clans. His reincarnation could also bring about a change in the relationship between the different villages as he tries to use his past experiences to create a more united and peaceful ninja world.
One possible plot could be that Naruto and Obito are forced to merge due to some dark jutsu gone wrong. Their combined form might initially struggle with two different sets of memories and personalities. Naruto's positive and determined nature could start to influence Obito's more cynical side. Eventually, they might use their combined powers to fight a new, powerful enemy that neither could have defeated alone.
One possible plot could be that Obito, having his own complex views on the ninja world, raises Naruto with a different set of values. He might train Naruto in more unorthodox ways, focusing on Naruto's raw power rather than the traditional ninja teachings. Naruto could grow up with a deeper understanding of the Uchiha's pain and Obito's motives, which could lead to a different path in his quest to become Hokage. For example, he might be more willing to question the village's decisions and seek real peace instead of just the village's prosperity.
One possible reason could be that Obito sees Naruto as a means to fulfill his own twisted ideals. Maybe he wants to shape Naruto into a weapon against the Hidden Leaf. Another reason might be that he feels some sort of connection to Naruto because of their shared experiences of being alone and ostracized in the past. He could also think that by taking Naruto as a baby, he can prevent Naruto from becoming the hero the village wants him to be, thus disrupting the balance in the ninja world.
One idea could be a story where Obito's mask holds a hidden power. For example, it could be a key to an ancient jutsu that was sealed away long ago. Another plot could involve a character trying to steal the mask for its significance in the ninja world.
One plot idea could be that Naruto discovers he has a baby unexpectedly. Maybe it was a result of a mission long ago where he unknowingly saved a woman who then had his child. The story could focus on Naruto trying to balance his new role as a father with his duties as Hokage. He would have to learn how to take care of the baby while still protecting the village.