In the 'Dolores Roach True Story', Dolores Roach is likely a real - life person on whom the story is based. She could be an ordinary woman with an extraordinary story, perhaps facing hardships, making mistakes, and learning from them throughout her life.
In a Witcher fanfiction, Harry as the Master of Death could become a sort of mentor to young Witchers. He could teach them about the different aspects of death and how to face it without fear. His ability to control death could also be used to study the dark magic and curses that are often associated with the monsters in the Witcher world. He might be able to reverse some of the more deadly curses or prevent the spread of dark magic that is harming the land and its people. Also, he could use his connection to death to understand the nature of the monsters better, as many of them are often related to death or the underworld in some way.
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.