Another plotline could involve Harry learning ancient magic related to elemental control. For example, he learns to master fire magic in an ancient way that is much more powerful than the regular spells. This newfound power helps him in battles against Voldemort's followers, as he can create massive walls of fire or send out fire - based attacks that are difficult to counter.
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.
Perhaps it's a magical crystal that enhances his ability to communicate with magical creatures. This would be quite unique as it would give him an edge in dealing with things like thestrals or unicorns, and open up new storylines in the fanfiction. Harry could use this crystal to understand their needs and gain their help in various situations.
Hermione could be a strong ally. She's smart and could help Harry research more about battle magic. Ron too, with his loyalty and courage, would be by Harry's side.
Obviously, Harry Potter is the main character as the title implies he is the one learning magic early. But there could also be Hermione and Ron. Hermione might be intrigued by Harry's newfound knowledge and Ron could be both a bit jealous and in awe of Harry's early magical abilities.
Overcoming limitations is also a theme. Since wandless magic is not the norm in the wizarding world as presented in the original books, Harry has to break through the traditional thinking. He has to deal with the doubts of others and his own insecurities. In some fanfics, he even has to face opposition from the Ministry of Magic who are wary of this unregulated form of magic.
Obviously, Harry Potter himself is a very important character as the story revolves around him learning family magic. His friends like Hermione and Ron might also be important as they could support or help him in his journey of learning this magic.
It could be an older relative who was thought to be dead but actually has been living in seclusion, waiting for the right time to teach Harry about their family magic. This relative might have a vast knowledge of the family's magical traditions and spells.
Maybe it could be an ancient magical being like a centaur. Centaurs are known for their knowledge of the stars and ancient magic. They might take Harry under their wing and teach him old magic that has been passed down through their generations.
One possible plotline could be Harry's internal struggle. As he learns dark magic, he has to battle his own moral compass. He might question the boundaries of good and evil. Another could be how his relationships change. His friends might be horrified or try to save him from the dark path.