Perhaps a unique fanfiction would be one where Harry Potter, the Master of Death, has to hide his identity from the Muggle world while still helping Sherlock. He could use magic in very subtle ways that Sherlock starts to notice but can't quite figure out. This would create a lot of tension and mystery in the story, as Sherlock tries to solve the mystery of Harry's abilities while Harry is trying to keep his magic a secret.
Well, in a fanfiction context, one way it could be possible is through some sort of magical crossover event. Maybe the magic in the Harry Potter universe has a strange interaction with the supernatural elements in Twilight. For example, the concept of the Master of Death in Harry Potter might give Harry some sort of power or ability that allows him to traverse into the Twilight universe and interact with its characters. It could also be that a powerful wizard or vampire in a fan - created story figures out a way to merge the two concepts, perhaps using some ancient magic or vampire lore that ties the two together.
One way it could work is if the Elder Wand's power was somehow equated to a Witcher's sword. The Elder Wand is the most powerful wand in the Harry Potter world, and a Witcher's sword is crucial for combat. Harry, as the Master of Death, could use the Elder Wand's power in a new way within the Witcher's world. He could perhaps use it to enhance his physical abilities like a Witcher's mutations do. Also, the Resurrection Stone aspect of the Deathly Hallows might have an impact on the Witcher's concept of death and resurrection, which is different from the normal human view in the Harry Potter world.
One way it could work is through the concept of alternate universes. In a Marvel - Potter crossover fanfic, the power of being the Master of Death could be seen as a sort of super - power. Harry could use his mastery over death to perhaps revive fallen heroes in the Marvel universe, like bringing back Bucky Barnes before his resurrection in the main storyline. This would create a whole new dynamic and set of relationships.
Well, in a RWBY fanfiction with Harry Potter as the Master of Death, one way it could work is through a cross - dimensional portal. Harry could be accidentally transported to the RWBY universe. His status as the Master of Death might give him unique powers there. Maybe he could use the Deathly Hallows' powers to combat the Grimm. For example, the Invisibility Cloak could help him in stealth missions against the Grimm, and the Elder Wand could enhance his combat abilities against powerful foes in RWBY.
One great fanfiction could be where Harry Potter, with his mastery over death, enters the world of Sherlock. Maybe he uses his magical abilities to solve some of the most complex cases that Sherlock is stumped on. For example, he could use Legilimency to read the minds of suspects, which would add a whole new dimension to the detective work.
A new character created specifically for the fanfiction could also be the Master of Death. This character could be a hybrid of a wizard and a vampire or werewolf, having the combined knowledge and abilities of both worlds. They could use their power to try and create a new order where wizards, vampires, and werewolves co - exist peacefully, using their mastery over death to enforce certain rules.
Well, in a fanfiction, one way it could work is if the concept of the Master of Death from Harry Potter was translated into the world of Soul Eater. Maybe the power of being the Master of Death could give the character an edge in dealing with the Kishin and the madness in Soul Eater. For example, the invincibility that might come with mastering death could help in battles against powerful foes.
He could interact with Thor. Harry might show Thor his Elder Wand and Thor could be impressed by its power. They could then team up for a battle against a common enemy, with Harry using his magic and Thor his hammer.
It could work if the Infinity Stones and the Deathly Hallows had some sort of connection. Since Harry is the Master of Death, he might be sensitive to the power of the Stones. The Avengers are dealing with the Stones' power, and Harry could bring his knowledge of dealing with powerful magical artifacts from his world. He could teach the Avengers about the consequences of misusing powerful items and how to safeguard against the corrupting influence of such power.
Harry could use his invisibility cloak to his advantage in Thor's world. He could spy on enemies or gather information without being detected. His ability to communicate with spirits as the Master of Death might also give him an edge. He could perhaps communicate with the souls of fallen Asgardians and learn their secrets.