Perhaps an old and reclusive wizard living in the Forbidden Forest could be Harry's mentor. This wizard has been practicing alchemy in secret for centuries and sees potential in Harry. He takes Harry under his wing and teaches him the secrets of alchemy that have been lost to most of the wizarding world.
Dumbledore could also be his mentor. Dumbledore was knowledgeable about many things, and it's possible that he had some understanding of alchemy. He could encourage Harry to explore alchemy as a way to enhance his magical abilities and use it for good.
Dumbledore could be his mentor. Even in death, his portrait or some magical remnant of him could guide Harry. Dumbledore was always knowledgeable about magic, and he could have left behind some secret teachings about mana absorption for Harry to discover.
An older, more experienced ninja from a hidden village that specializes in needle - based jutsu could become his mentor. This ninja could have unique insights into the art of using needles, teaching Naruto different formations and strategies for using them in battle.
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.
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.
Perhaps the Sage of Six Paths could be his mentor. Since the Sage has great knowledge and power, he could guide Naruto in understanding and using soul magic. Another option could be Jiraiya. Even in death, Jiraiya's spirit could communicate with Naruto and teach him about soul magic through visions or dreams.
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.
It could be an old hermit living in the forest who has a deep connection with nature and Mokuton. This hermit has been observing Naruto from afar and decides to come forward to teach him. The hermit could have a different approach to using Mokuton, one that is more in tune with nature's balance, which Naruto would find both challenging and enlightening.
Luna Lovegood's father, Xenophilius Lovegood. He's an eccentric but knowledgeable wizard. He could introduce Harry to some very unique forms of magic and different ways of thinking about the magical world. Harry would get to explore some really out - there magical concepts under his mentorship.