Lupin could be a very interesting mentor. He could teach an apprentice about dealing with their own inner demons, as he has his own struggles with being a werewolf. He could also pass on his knowledge of defensive magic and how to use magic in a more practical and street - smart way.
It could be Hagrid. He has a lot of knowledge about different magical creatures and skills. He might introduce Harry to marksmanship as a way to protect himself better in the wild. Hagrid could show Harry how to make his own arrows and teach him the basics of aiming and shooting.
Perhaps Hagrid could be his mentor. Hagrid has a lot of knowledge about magical creatures and their parts, which could be useful for Harry in his blacksmithing. He could show Harry how to use things like dragon scales or unicorn hairs in his creations.
Perhaps Neville Longbottom could take on this role. He has shown great courage throughout the series. His experiences with loss, like the death of his parents, could lead him down a path where he becomes the apprentice of death, using his newfound powers to protect others and bring a new balance to the wizarding world.
Dumbledore could be his mentor. He has vast knowledge and experience, and he would likely see the potential in Harry being an Animagus. He could guide Harry through the difficult process of learning to control his transformation and use his Animagus form for good.
Maybe an old wizard who has been secretly studying the combination of different magical universes for years. This wizard could have made some initial attempts at building a lightsaber - like weapon in the past and can share his experiences and mistakes with Harry. He would teach Harry about the balance between the light and dark side of this new form of magic, just like the Jedi teachings in Star Wars.
Snape is a likely candidate. He is a skilled legilimens in the original series, and his relationship with Harry is already complex. Teaching Harry legilimency could be a way for Snape to either help or hinder Harry, depending on his motives.
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.
It could be a powerful wizard from Asia who has long studied both magic and kung fu. This wizard has decided to pass on his knowledge to Harry Potter, seeing his potential.
Maybe Dumbledore in a more in - depth way. In the original series, Dumbledore was somewhat mysterious in his teachings. Here, he could really focus on Harry's growth and give him proper training, perhaps in the art of divination and understanding prophecies better.
Dumbledore would be an excellent choice. He is extremely powerful and wise. His apprentice would learn not only about powerful spells but also about the greater good and moral responsibilities that come with magic.