His friends from the original story could also be main characters. For example, Hermione might be there, constantly worried about Harry's dangerous dragon - riding adventures but also helping him with research about dragons and their habitats.
There could be the mafia boss. He might be a tough and shrewd character who takes Harry under his wing at first but later becomes suspicious of Harry's actions. Another main character could be Harry's best friend in the mafia. They go through all the dangerous and exciting adventures together, and this friend might have different views on the mafia life compared to Harry.
There could be a doll collector named Alice. She might be Harry's friend or rival, always competing to get the best dolls in the story.
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.
Ron could also be there. He might be initially scared of the dragon but then grows to like it. His loyalty to Harry would make him support Harry's decisions regarding the dragon, even if they seem risky. For example, when they need to hide the dragon from the Ministry of Magic, Ron would be right there with Harry, coming up with crazy but effective plans.
It could be the clan leader. He might play a crucial role in guiding Harry or challenging him within the clan.
There could be Hermione. Since she's always interested in learning new things, she might be involved in Harry's exploration of mysticism.
There could be original characters created specifically for the fanfiction. For example, a wizard who has some knowledge about the origin of the Omnitrix and tries to guide Harry. This character could be an old hermit - like wizard who has been studying interdimensional objects for years. Also, there might be a villain who is after the Omnitrix, someone who wants to use its power for evil purposes and constantly hunts Harry down.
There could be a range of characters. For instance, there might be a powerful Sith Lord who sees Harry as a threat to the traditional Sith ways because of his neutral stance. On the other hand, there could be a young Jedi who is initially suspicious of Harry but later comes to understand his unique position. There might also be some side characters, like smugglers or bounty hunters, who get involved in the story because of Harry's actions and the power struggle around him.
Perhaps there is a female alien diplomat who tries to negotiate with Harry to stop his tyrannical rule. Also, there could be a group of human astronauts who end up in his space domain and have to either submit to his rule or try to escape. Another character could be an old space wizard who has knowledge of a power that could dethrone Harry and is hunted by his forces.