It could be an ancient shinobi who has mastered some kind of death - related jutsu and has ascended to a higher state of being. This shinobi might have been forgotten in the history of the ninja world.
One possibility is that Naruto meets the personification of Death in this fanfiction. They could have an intense conversation about the value of life and death, considering Naruto has seen so much death and destruction in his own world. And perhaps Death gives Naruto a new mission related to the balance between the living and the dead.
Obviously Naruto himself. And from the 'Death' side, the Shinigami is very likely to show up. It's the most iconic death - related character in that universe.
It could be the First Hokage, Hashirama. Naruto would be amazed by his powerful Wood - Style jutsu and his ability to create the village. He could learn about the original ideals behind the Hidden Leaf from Hashirama.
The 'reader meets death sans fanfiction' can offer a lot in terms of character development. For the reader character, they may grow in terms of their understanding of life and death. Death Sans himself can be presented in many ways, from a menacing figure to a more complex and sympathetic being. The fanfiction also has the potential to create a really immersive world that blurs the lines between the known and the unknown.
Bellatrix Lestrange is also a likely candidate. Her encounter with Hermione would be extremely intense. Given her loyalty to Voldemort and her rather unhinged nature, she would likely pose a serious threat to Hermione, but Hermione might also find a way to use Bellatrix's madness against her.
In a fanfiction where Naruto meets an AI, it could be an interesting scenario. Maybe the AI has some unique powers or knowledge that Naruto can use to his advantage. It could also lead to some comical situations as Naruto tries to understand this strange new technology.
In many Naruto fanfictions where Naruto meets Kushina and Minato, it often creates a very emotional scenario. Naruto finally gets to meet his parents who he has only heard about. It could be a heart - warming reunion where they share stories, hugs, and Minato and Kushina get to see how far Naruto has come.
Teamwork is also a big theme. Despite being the same person in essence, the two Narutos often have to learn to work together. This can be challenging as they may have different personalities due to their different experiences. For example, one Naruto might be more cautious while the other is more impulsive. And the theme of friendship also plays a role, as they are, in a way, their own best friends in these stories.