Lily and James Potter could still be important characters. Their friends might be reflecting on their sacrifice and the loss of their son. Maybe through flashbacks or the way their friends remember them, they would play a role in the story.
Voldemort would surely be a main character. With Harry dead as a baby, Voldemort's actions and how he further consolidates his power would be a major part of the story.
Sirius Black could be a central figure. He was Harry's godfather and his anger and grief over Harry's death could drive his actions. He might be on a quest for revenge against Voldemort or trying to protect those that Harry was supposed to protect in the original story.
Megatron could also be a major character. Maybe he views Harry Potter as a threat to his power and plans. He could try to either destroy Harry or use his unique abilities for his own evil purposes. And then there could be Bumblebee, who befriends Harry and helps him communicate with the other Autobots, since Bumblebee is often good at making connections.
Madame Maxime, the headmistress of Beauxbatons, could play an important role. She could be a mentor to Harry, guiding him in his new magical studies at Beauxbatons. She might also be involved in some sort of conflict or mystery that Harry has to help her solve, which could involve the relationship between Beauxbatons and other wizarding schools or some ancient magic within the school.
The adoptive family members could be main characters. For instance, the adoptive parents who might have their own unique personalities and backstories. They could be former Aurors who retired early to focus on raising Harry. There could also be an older sibling in the adoptive family, who might initially be jealous of Harry but later becomes his closest ally.
There could be new characters introduced in this fanfiction. For example, a local wizard in the place where Harry moves to could become a main character. This wizard could initially be suspicious of Harry but later become his friend. Also, a magical creature that Harry befriends in his new location could be a central figure. It could help Harry adapt to his new surroundings and teach him things about the local magic that he didn't know before.
Dumbledore could also play a role even if Harry has left Earth. Maybe his spirit or some form of his magic reaches out to Harry from afar, guiding him on his journey and providing him with cryptic advice to face the challenges in outer - space - related magic.
Hermione would definitely be one of the main characters. She could be the one to research all about braces in the wizarding library to see if there are any magical implications or ways to make the process easier for Harry. And she would offer him lots of advice and support.
There might also be a love interest for Harry. This character could be someone who is also involved in the dangerous world of assassination in some way. She or he might be a fellow assassin or someone who is on the periphery of that world but still gets drawn into Harry's life, adding a romantic sub - plot to the story.
Hermione could be a main character. She might be the one who helps Harry understand and control his gorgon powers. For example, she could research in the library to find ancient spells or potions that can suppress the gorgon - like effects when they are unwanted.
Another character could be a fellow inmate who has been in jail for a long time. He befriends Harry and teaches him the ropes of surviving in jail.
There could be some of Harry's friends who might play a role. They could be the ones who encourage Harry to meet the Queen or who give him advice on how to behave in front of her.