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.
The idea of the Master of Death in a Harry Potter - Naruto fanfiction might involve a crossover of abilities. For example, a character like Harry Potter who has mastered death could potentially teach the ninja in Naruto's world about different ways of facing mortality. It could also lead to some unique battles where the rules of both universes are combined. Like, using magic against chakra - based attacks while having the immunity to death that comes with being the Master of Death.
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.
In this fanfiction, Naruto is the central figure. However, other important characters could include Kakashi. Kakashi, being Naruto's former teacher, would be interested in Naruto's new power and would try to guide him on how to use it responsibly. There might also be new characters introduced, like some sort of death spirit or deity that has a connection to Naruto's power. These new characters could play a role in either helping or hindering Naruto as he tries to master his role as the master of death.
Well, it could be about Naruto obtaining the power of a master over death. Maybe he can bring back the dead or has the ability to control deathly forces. He might use this new power to protect his village from greater threats that normal ninja skills can't handle.
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.