Maybe it's a member of a long - lost ninja clan. This clan member could have some special skills and knowledge that they want to pass on to Naruto. They choose to train him in secret to protect their clan's secrets from falling into the wrong hands.
There's a possibility that it's a spirit or a ghost of a great ninja from the past. This entity might have chosen Naruto because of his strong will and determination. Since it's a spirit, it has to train Naruto in secret as its existence might not be acceptable in the ninja world or might attract unwanted attention from dark forces.
Perhaps a fallen god who, despite losing his status, still has the knowledge of how to train a God of Destruction. This character could have his own motives for training Naruto, like regaining his former glory through Naruto's success.
Well, it could be about Naruto finding a hidden master who teaches him unique jutsus in secrecy. Maybe this master has a special connection to an ancient ninja clan and imparts knowledge that no one else in the village knows.
It could be an unknown ninja from a far - off village who specializes in Storm Release. This ninja might have heard of Naruto's potential and decided to train him.
It could be an old, retired ninja who lives on the outskirts of the village. This person has seen the hardships Naruto faces and decides to take him under their wing.
There could be new antagonists created specifically for this fanfic. Maybe some dark forces that were sealed away by Asura long ago are awakened. They would oppose Naruto because they see him as a threat to their resurrection or their evil plans.
It could be some rogue shinobi who are after Pakura's knowledge. Since she's training Naruto, they might see it as an opportunity to get to both of them and steal their techniques. Maybe they were once her enemies or just power - hungry individuals.
There's a chance it could be a powerful house - elf. House - elves are known for their own brand of magic, and one particularly rebellious or enlightened house - elf could decide to train Harry in magic that is more about stealth and quick - thinking. After all, house - elves have to be very sneaky to avoid detection by their masters most of the time, and this kind of skill could be very useful for Harry in his battles against Voldemort.
Maybe it's a former Death Eater who has seen the error of his ways. He wants to atone for his past sins by training Harry to defeat Voldemort. He has inside knowledge of the dark side's tactics and uses this to make Harry a more formidable opponent.
The Hokage, like Tsunade or Kakashi, could be the ones training Naruto. They know Naruto well and have a lot of knowledge and skills to pass on.
It could be someone from Guts' world who follows him into Naruto's world. Maybe one of the Apostles. They are powerful and evil beings. Their presence in the Naruto world would be a huge threat and could be a great antagonist as they would have powers that are different from what Naruto is used to dealing with.