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.
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.
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.
As far as I know,'Dream of the Red Chamber' was not called 'Dream of the Red Chamber' when Cao Xueqin was alive. Instead, it was called 'Story of the Stone.' This name came from the nickname of the character Stone in Cao Xueqin's novel. Cao Xueqin had not died when he wrote this novel, so after his death, this novel was called "Dream of the Red Chamber" by others and gradually spread and accepted.
One interpretation could be that it refers to the moment when a life - threatening event occurs and changes the course of a life story. For example, a serious illness diagnosis can be seen as 'death' in a sense meeting the person's life story, as it forces them to reevaluate everything and can lead to a completely different future than what was originally planned.
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.
It's interesting because it can explore deep philosophical ideas about death and existence in a fictional and accessible way. Through the encounter between the reader and Death Sans, different views on life after death or the meaning of life can be presented.
In a fanfic where Harry Potter meets Death, one possibility is that Death offers Harry a choice. Maybe it's a choice between saving his loved ones or sacrificing something of great value to him. This could lead to a moral dilemma for Harry, testing his character in ways not fully explored in the original series.
Another possibility is that Death is being controlled or influenced by a greater cosmic entity. In this case, that entity could be the main antagonist. It uses Death as a tool to test or destroy the Avengers. The Avengers would then have to not only deal with Death but also find a way to face this more powerful and mysterious entity behind Death's actions.