There could be a local wizard who becomes Harry's mentor in a way. Since Harry is on his own, he might seek guidance from someone outside of his usual circle, like an old hermit - like wizard who has a lot of knowledge about the more hidden parts of the wizarding world.
Hermione could be important. She might help Harry in his journey as a mage, using her intelligence to research and understand new magical concepts that Harry discovers. Ron could also be there, providing his loyalty and friendship, perhaps even joining Harry in some of his mage - related adventures. And then there could be a new character, a powerful mage who becomes Harry's mentor and teaches him the deeper secrets of magic.
Hermione could be one. She might follow Harry to the island to help him out. Another could be Ron, his loyal friend. Or it could be some new characters created specifically for this fanfiction, like a wise old wizard who has been living on the island for centuries.
Hermione could be one. She might follow Harry to Australia to continue their adventures together. She could be interested in studying the different magical plants in Australia and how they can be used in potions.
There could be an old French wizard named Alphonse. He might be a mentor to Harry, passing on his knowledge of ancient French magic. Also, a young French witch named Celeste could be a main character. She and Harry could develop a friendship or more, and she could be part of Harry's new life in France, like helping him understand the French magical education system which is different from Hogwarts.
Besides Harry, there could be a character who is a descendant of an ancient elemental magic family. This character might initially be a rival to Harry in mastering the magic but later becomes an ally. Also, Dumbledore could play an important role. He might be secretly guiding Harry in his journey of learning elemental magic, using his own deep knowledge of the wizarding world's secrets.
Hermione would probably be involved. She could help Harry research about the island, like its history and any magical properties it might have.
Hermione could be important. She might be the one who encourages Harry to drop his mask, or she could be the first to notice the change when he does. Another could be Ron, as Harry's best friend, his reaction to Harry dropping his mask would be significant.
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.
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.
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.