One possibility could be that Harry uses his mastery of death to communicate with the spirits in the Avatar world, similar to how he could interact with the dead in the Potterverse. He might gain new powers related to the elements, as his connection to death gives him a deeper understanding of the balance in nature, which is crucial in the Avatar world.
One possibility could be that Harry uses his mastery over death to bring back some fallen Avengers. He might use his knowledge of powerful magic to enhance the Avengers' abilities. For example, he could place protective spells on them similar to the ones used in Hogwarts.
Harry Potter himself could be a potential Avatar of Death. Given his connection to Voldemort and all the death and sacrifice he has witnessed, he might develop some sort of connection to death in a more profound way. It would be a very dark and different take on his character, but it could also add a whole new layer of depth.
One possible fanfiction idea could be Harry Potter, after becoming the Master of Death, somehow gets transported to the Dragon Age world. He might use his knowledge of magic and his new - found mastery over death to change the course of events in the Dragon Age. For example, he could use the Deathly Hallows in ways that are unheard of in the Dragon Age, like using the Resurrection Stone to bring back important characters who died prematurely in the Dragon Age lore.
In such a fanfic, Harry Potter as the Master of Death would bring a whole new dynamic to the Justice League. His magical abilities are different from the superpowers of the League members. He might have to adjust to the more scientific and modern world of the Justice League while still relying on his old magical knowledge. And the League would have to learn to work with his rather unorthodox methods.
It could be about a character from the 'Harry Potter' world, perhaps Harry himself as the master of death, somehow ending up in the 'Inuyasha' universe. Maybe he uses his magic skills and knowledge of death to help Inuyasha and his friends in their battles against demons.
Another aspect could be the character development. If a character becomes an Avatar of Death, it would change their personality and motives completely. They might start seeing life and death in a very different way, perhaps even challenging the moral and ethical boundaries that are so important in the Harry Potter series. For instance, they could question the value of sacrificing oneself for the greater good when they have a new perspective on death.
Well, in such a fanfiction, perhaps Harry Potter, being the Master of Death, has some knowledge or power that Thor needs. For example, if there is a dark force in Asgard that is related to death in some way, Thor might seek Harry's help. Their encounter could lead to a cultural exchange too, with Harry learning about Norse mythology and Thor about the wizarding world.
It could be an epic crossover. Harry, with his mastery over death, might bring some unique magic to the Avengers. Maybe he could use his skills to revive fallen heroes or protect the world in a different way than the Avengers are used to.
In fanfiction, Harry as the Master of Death could also start a new school of magic in the Marvel universe. This could attract both mutants and regular humans with an aptitude for magic. It would be a place where they could learn about the different aspects of magic, especially those related to life and death. This would not only expand the magical community in the Marvel universe but also create a new generation of heroes and magic - users who have a different understanding of death and its implications.
One possible fanfiction could be where Harry, as the Master of Death, has a secret connection with Daphne. Maybe Daphne discovers Harry's true power and they form an alliance to fight the remaining dark forces in the wizarding world. They could go on adventures together, using Harry's new - found status to bring about a new era of peace and magic.