In fanfic, it could be that Arcturus Black was a part of a secret society or had a mission that was related to the Potter family. He might have made some kind of pact or arrangement that made Harry his heir. It could also be a case of alternate timelines or universes where the normal rules of inheritance don't apply. Maybe in this particular fanfic's world, the magical laws are different and Arcturus Black's connection to Harry as an heir is a new and exciting concept.
In the fanfiction, Arcturus Black is likely an important figure in the wizarding world. He might be a powerful and knowledgeable wizard from the Black family. Maybe he has a long - standing reputation for his magical abilities and wisdom, which is why he is chosen to train Harry Potter.
Arcturus Black might be portrayed as a wizard with a deep understanding of ancient magic in the fanfiction. He saves Harry Potter perhaps because he sees in Harry the potential to defeat the dark forces that are plaguing the wizarding world. His character could be a blend of mystery, power, and a sense of moral obligation towards Harry.
The Death Eaters who still hold onto the old pure - blood supremacist views could be main antagonists. They might see the Heir of Arcturus Black as a threat to their ideology, especially if the Heir is trying to change the Black family's ways.
It might also mean that Harry inherits certain magical abilities or knowledge. Since Arcturus Black may have had his own set of skills and secrets, Harry as his heir could gain access to them. This could change the way Harry approaches challenges in the wizarding world, like fighting against Voldemort. He might have new spells or strategies at his disposal. Also, it could affect his relationships with other characters. Some might view him differently, either with more respect or with envy. And it could open up new plotlines in the fanfic, such as Harry having to deal with the expectations and responsibilities that come with being Arcturus Black's heir.
One idea could be exploring how the Heir of Arcturus Black discovers hidden Black family magic. Maybe they find a secret chamber in the Black family home filled with ancient spells and artifacts. Another could be their interaction with Harry Potter. They might team up to fight against the resurgence of dark wizards in Hogwarts.
In some fanfictions, it might be possible for Harry Potter to be depicted as the heir of Grindelwald. Fanfiction allows for a lot of creative interpretations. However, this goes against the original canon where there is no such indication.
In the fanfiction world, Harry might be the heir to Hogwarts if there was some sort of magical pact made generations ago. For example, perhaps one of his ancestors had a special role in the creation or protection of Hogwarts, and this role was passed down through the family. It could also be that his unique magical abilities, like his ability to resist Voldemort's attacks from a young age, are signs of a deeper connection to Hogwarts itself. This connection could be what designates him as the heir in the fanfiction's plot.
In some Harry Potter fanfiction, Arcturus Black might help by using his family's influence. For example, the Black family has a long history and certain political power within the wizarding world. He could use this to protect Harry Potter from some of the more dangerous pure - blood supremacist groups. He may also offer Harry guidance on the complex politics and social hierarchies of the wizarding world, drawing on his own experiences as a member of an old and influential family.
A major theme is the uncovering of secrets. In many of these fanfictions, Arcturus Black holds some sort of secret knowledge related to the wizarding world, perhaps something about the Dark Arts or the history of Hogwarts. Harry, being the curious and brave character he is, is drawn into this web of secrets. And also, friendship is a theme. Despite the age and family differences, a bond of friendship or a deeper connection forms between them.