Obviously, Harry Potter is the main character as it's centered around him being the Immortal Master of Death. He would be a more powerful and perhaps more complex version of himself from the original series.
One interesting plotline could be Harry's journey to fully understand and master the power of being the Master of Death. He might face challenges from dark wizards who are envious of his newfound status.
One possible fanfic could involve Harry Potter, with his newfound status as the Master of Death, being transported to Asgard. He might use his skills and powers to face the challenges there. For example, his ability to master death could give him an edge in battles against Asgardian foes. Maybe he'd form an alliance with Thor, and together they'd fight against a common enemy like Loki who has some new, more dangerous scheme.
One interesting plotline could be Harry fully exploring and mastering the powers that come with being the Master of Death. He might use these newfound abilities to reshape the wizarding world, like eliminating dark magic at its roots.
It could be that the character who is the Master of Death in the fanfic is seen as a prophesied figure in the Warcraft universe. Since the Master of Death has conquered death in the Harry Potter world, in Warcraft they might be seen as a being who can bring new life or restore balance. For example, they could use their power to heal the war - torn lands in Warcraft, much like how they might have used their power to protect or revive in the Harry Potter realm.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
Well, Harry could find that his Deathly Hallows are perceived very differently in Asgard. His Invisibility Cloak might be seen as a powerful magical artifact like the ones in Asgard. His Resurrection Stone could be a source of great fascination as Asgardians have their own beliefs about the afterlife. And his Elder Wand might be seen as a weapon of great power, perhaps on par with Mjolnir.
A common theme is power and its consequences. Since Harry is the master of death, he has a great deal of power, and fanfics often explore how he uses or is tempted by it. Another theme is the blurring of boundaries between the known magical world in Harry Potter and the added supernatural elements.
One possibility could be that in this fanfiction, Harry Potter uses his mastery of death to have unique abilities in Skyrim. Maybe he can't truly die in the Skyrim world, and this gives him an edge in battles against dragons or other foes. He could also use his knowledge from the wizarding world to manipulate magic in Skyrim in ways that are different from the native mages.
Yes, there might be. Some creative fanfiction writers like to blend different fictional universes. They could create a story where the concept of 'Harry Potter being the Master of Death' is incorporated into a 'LotR' - themed fanfic. It could be an interesting crossover with unique plotlines.