In such a fanfic, a possible plot is that Harry, being raised by Voldemort, has a completely different understanding of the prophecy. He may think that he is meant to rule the wizarding world alongside Voldemort rather than defeat him. As he interacts with other characters like Draco Malfoy, who might be his ally in this dark upbringing, he gradually realizes that the path he's on is not what he truly wants. This could then lead to a transformation in Harry, where he decides to break free from Voldemort's influence and fight for the side of good, but with a very different set of skills and knowledge compared to the Harry in the original series.
Well, perhaps in this fanfic, Harry is raised by Voldemort to be his ultimate weapon against the wizarding world. He's trained to be extremely powerful in combat magic. But as he grows, he starts to notice the fear and suffering Voldemort causes. This could lead to an internal struggle within Harry, between his loyalty to the man who raised him and his own sense of right and wrong. Also, there could be a plot where Harry uses his unique position to secretly undermine Voldemort's plans from within his own ranks.
One plot idea could be that Harry discovers unique magical creatures that exist only in space. For example, there could be a species of floating, star - like creatures that can communicate through light pulses. Another idea is that he has to learn to control his magic in zero - gravity, which presents a whole new set of challenges compared to on Earth. Maybe he also forms an alliance with alien races who are also magically inclined and together they fight against a common dark force that threatens both the wizarding world and the galaxies.
Perhaps the monster has a vendetta against certain wizarding families, and as Harry grows up under its care, he is caught in the middle of this conflict. He has to decide whether to side with the monster or try to find a way to reconcile the differences. This could lead to a whole new set of adventures and challenges for Harry.
One plot idea could be that they discover they are mates due to a powerful magical bond. They then have to hide this fact from the wizarding world which is in the midst of a war. Harry might try to convince Voldemort to abandon his dark ways, using their newfound connection as a way to appeal to his better nature.
One idea could be that his grandparents, being more experienced in the wizarding world than the Dursleys, start teaching Harry magic early on in a more positive and nurturing way. Maybe they introduce him to family secrets and spells that have been passed down through generations.
One interesting plot idea could be that Snape raises Harry with a strict but fair hand. He teaches Harry about the Dark Arts not to encourage him to use them, but to make him fully aware of the dangers. Harry might grow up with a more nuanced view of Snape from the start, and their relationship could be a mix of father - son dynamic and teacher - student. For example, Snape could train Harry in potions from a very young age, leading Harry to become a master of potions himself much earlier than in the original story.
One plot idea could be Harry growing up in a very different environment. Instead of the Dursleys, he's raised in the dark wizarding world under the influence of Bellatrix and Voldemort. He might struggle with his identity, having the blood of both a hero (in a way, as he's related to Harry Potter in the original story) and two of the darkest wizards. He could be trained in dark magic from a young age but have an inner conflict due to some latent goodness in him.
A plot idea might be that the wizarding world initially turns against Harry when they find out he's Voldemort's heir. However, Harry uses this new identity to his advantage by understanding Voldemort's motives better. He could start to study dark magic not to use it for evil, but to find ways to counter it more effectively. For example, he might find a way to use some of the spells Voldemort created to protect the school instead.
One idea could be that Harry, as Voldemort's soulmate, tries to use their connection to redeem Voldemort. He might show Voldemort memories of love and kindness in an attempt to change his dark ways.
One idea could be that his grandparents are very strict about magic, so Harry has to secretly practice it. He might discover some old family magic that his grandparents were unaware of.
One idea could be that Sirius teaches Harry all about his family's history in a much more in - depth way than Dumbledore ever did. He might show Harry secret passages in the Potter family home. Another plot point could be Harry learning advanced magic earlier under Sirius' tutelage, like Patronus charm at a younger age.