Harry Potter himself could be the main character. Since he has faced death so many times in the original series, it would be interesting to see how he interacts with the concept of Death from the Endless.
It might involve the Deathly Hallows taking on new significance in relation to the Endless Death. For example, the Elder Wand could be seen as a tool that even Death from the Endless might be interested in. The Resurrection Stone could have a connection to bringing back those who have passed on in a way that goes beyond the normal magic in Harry Potter, influenced by the nature of Death in the other fictional context.
It could be a Slytherin student who has always been drawn to the darker aspects of magic. They might have grown up in a family with a history of dark magic, and this has influenced their view of death as something to be embraced rather than feared.
It could be a character who was once on the side of the light but was corrupted in some way and became Death's Knight. Maybe Neville Longbottom, who has always been underestimated. His transformation into Death's Knight could bring a whole new perspective to his character, as he battles his inner demons.
It could be Voldemort again, but in a more powerful and death - centered form. Maybe he has found a way to fully embrace the power of death and become an even greater threat. Since Harry is the champion of death, it would be a battle of two opposing forces related to death.
Death, of course, would be a major character as it's training Harry. Maybe Dumbledore could also play an important role. He might be the one who initially guides Harry to this encounter with Death or acts as a moral compass during the training.
It could be an unknown character who has been in hiding all this time. Maybe she was once a powerful witch who made a pact with death and became its representative in the wizarding world.
Hermione could be a great main character in an elementalist fanfiction. Her intelligence and thirst for knowledge would make her eager to learn about and master elemental magic. She could be the one to discover a long - lost elemental spell book and start exploring the powers.
It could be a new student at Hogwarts who has a natural talent for dealing with beasts. This student could be from a family that has always had an affinity for magical creatures but has kept it a secret until now.
A Muggle - born mouse. It could be a unique concept where a mouse from the Muggle world somehow ends up in the wizarding world. It would be interesting to see how it copes with all the magic and the rules of the wizarding society.
Neville could also be a main character in this type of fanfiction. He could stumble upon a clockwork - related mystery in the Hogwarts gardens. Maybe there are some clockwork - powered plants that are acting strange. Neville, with his new - found confidence, takes it upon himself to solve the mystery and in the process, uncovers a deeper secret about Hogwarts and its connection to clockwork magic.