Snape is a character that could play a major role. Given his past with dark magic, he might be a reluctant mentor to Harry, guiding him in understanding the power and danger of it. Dumbledore, too, would likely be in the story, as he is always concerned about Harry's well - being and actions.
Besides Harry, Hermione is a key character. She can research and find loopholes in the Ministry's laws and regulations. Ron is there too, providing muscle and emotional support. Neville Longbottom could also be involved. He has grown into a strong and brave wizard, and he might join Harry's cause. Additionally, Luna Lovegood with her unique perspective on magic could offer different ways of approaching the conflict with the Ministry. Some fanfictions might also include Sirius Black, if set in a timeline where he didn't die, as he has a history of opposing the Ministry's unjust actions.
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.
There could be Hermione. Since she's always interested in learning new things, she might be involved in Harry's exploration of mysticism.
Some of the teachers at Hogwarts could be involved as well. Professor McGonagall, for instance, might teach Harry some advanced Transfiguration magic. And Snape, before his death, could have left behind some notes or knowledge about Potions - related magic that Harry discovers and uses to learn more powerful potions and the magic associated with them.
Hermione is often a common character. She might help Harry research and understand the ancient magic, using her vast knowledge from the library. Ron could also be there, providing his loyalty and sometimes comic relief during Harry's journey of learning ancient magic.
Severus Snape is also a popular choice. His complex character and vast knowledge of magic can create interesting dynamics when he meets characters from different magical worlds. He could either be a mentor or an antagonist in these crossovers.
Ron could also be there. He might be in charge of something like the Gryffindor Quidditch team or be a part - time teacher teaching the younger students about the magical history of the Weasley family's inventions. His humor and loyalty would be a great addition to the story.
Harry Potter himself is often involved. Writers might explore how he could use the magic eye in different ways to solve mysteries or fight against the dark forces. Hermione would also be a common character. She could study the magic eye and try to understand its mechanics and magic better.
Obviously, Harry Potter himself is the central character. Then there's Dumbledore, who might be even more vigilant in protecting Harry earlier due to his early magic display. Hermione could also be involved, perhaps being even more intrigued by Harry's early magical experiences and trying to study them.
Ron could also be there. He might be a bit jealous at first if Harry's adopted family is wealthier or has more influence in the wizarding world. But later, he would support Harry as always.