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.
In such a fanfiction, Harry might discover ancient spells that give him an edge in battles. For example, he could learn old defensive magic that can protect Hogwarts in a more powerful way than the existing spells. He might also use old magic to communicate with long - lost magical creatures that hold secrets of the wizarding world.
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 a wizard who has traveled to far - off lands where voodoo is practiced could teach Harry. This wizard could have seen the power of voodoo and thought it could be useful in the fight against the Dark Arts in the wizarding world of Harry Potter.
The Death Eaters could still be the main antagonists. If Harry starts learning soul magic, they might see it as a threat and try to stop him.
Obviously, Harry Potter is a main character as the title suggests. He is the one who is on the journey of learning wandless magic. Hermione Granger could also be an important character. She might be helping Harry research about wandless magic, using her vast knowledge from the library.
It could be Hagrid. He's always had a connection to strange and powerful things. Maybe he stumbles upon the knowledge of nen during one of his trips in the magical forest and then passes it on to Harry.
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.
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.
Yes, it can be considered dark and controversial. Blood magic in the Harry Potter universe is often associated with dark and unethical practices. It goes against the principles of the good magic that Hogwarts teaches. For example, the use of blood in dark spells is seen in the actions of Voldemort. In fanfiction, having Harry Potter learn blood magic is a deviation from his character's usual moral stance, which can stir up a lot of debate among fans.
Some good 'Harry Potter' old magic fanfictions include 'The Enchanted Artefacts of Old'. This one is about finding magical artefacts that are filled with old magic. The characters go on a quest to discover these items and understand their powers. 'The Old Magic in the Forbidden Forest' is another. It takes place mainly in the Forbidden Forest, exploring the ancient magic that lurks there. There are strange creatures and hidden secrets related to the old magic. And 'The Dark Old Magic' is a bit darker, showing how some old magic can be corrupted and misused in the wizarding world.