Well, he'd probably bring his magical skills to the table. He could use spells for things like quick repairs on the ship or for defense against unexpected threats in space. His ability to think on his feet, which he developed during his battles at Hogwarts, would be a great asset in Starfleet's various missions.
It could be really interesting. Maybe Harry Potter, with his skills in magic and leadership from Hogwarts, would bring a whole new dynamic to the Starfleet. He might use magic in unexpected ways to solve problems that the regular Starfleet protocols can't handle. For example, he could use a spell to quickly repair a damaged ship or to communicate with alien species that don't respond to normal means of communication.
Perhaps the main antagonists could be a rogue Starfleet officer or a group of them who are jealous of Harry's rapid rise to captaincy. They might be secretly in league with some space pirates or other criminal elements. They could try to frame Harry for various crimes or undermine his leadership at every turn. Also, there could be an ancient alien entity that is awakened by the presence of Harry's magic and sees it as something that must be destroyed to preserve the order of the universe.
If Harry Potter were to join the Marines, it would be a very interesting combination. He might struggle with the muggle technology at first, but his determination and courage would help him adapt quickly. His magical abilities could potentially give the Marines an edge in combat situations, like using magic to disarm enemies or create barriers. Also, his experience with teamwork from his time with Ron and Hermione could enhance the camaraderie within the Marine unit.
His Hogwarts training taught him about strategy. In Quidditch, he had to plan his moves and anticipate the actions of others. This would be useful in Starfleet missions. Also, his Defense Against the Dark Arts skills could be adapted to handle dangerous situations in space.
Hermione would likely be one of his allies. She's intelligent and has a strong sense of justice, always having been interested in the rights and wrongs of the wizarding laws. She could provide Harry with valuable research and advice.
There could be the leader of the coven. This person might be an old and powerful witch or wizard who has been in the coven for a long time. They would be the one to guide Harry into the ways of the coven.
In these fanfictions, Harry's character transformation is quite drastic. He loses his sense of morality. His loyalty, which was once towards his friends and the good side, now shifts to Voldemort. He becomes more power - hungry, willing to do whatever it takes to gain more power, including using dark magic without hesitation. He also becomes more secretive and manipulative, using his former relationships to further Voldemort's goals.
Well, he'd probably bring his magical skills to the military. Maybe he could use spells for things like reconnaissance. For example, he could use the invisibility cloak to spy on the enemy, or use the Patronus charm to send messages in a secure way. His quick thinking and combat experience from fighting Voldemort would also be useful in military strategies.
Well, it could be about Harry Potter exploring a new world of magic within the coven. Maybe he discovers new spells or magical abilities that are different from what he learned at Hogwarts. He might also meet new and interesting characters who have their own unique magical practices.