I think Dumbledore might be a candidate in the fanfiction. He's the most powerful wizard of his time and could offer Harry deep insights into magic, history, and the fight against the dark side. Also, Luna Lovegood could be a tutor in her own strange but brilliant way. She has an unconventional view of magic that might open new perspectives for Harry. Then there's Sirius Black. He could teach Harry about the wizarding world outside of Hogwarts and some spells and tricks he learned during his time as a fugitive.
One important area would be Occlumency and Legilimency. Since Harry has had issues with his mind being invaded in the past, the tutors could help him master these skills. They could also teach him about different magical cultures. For instance, about the goblin culture, their magic with metals and treasure - related spells. And of course, the tutors could enhance Harry's knowledge of magical creatures, how to interact with them, and use their abilities to his advantage.
One potential private tutor could be a retired Auror. They would have a wealth of knowledge about the Dark Arts and how to combat them. They could pass on their skills in dueling and detecting dark magic to students like Harry Potter.
It could be an old, retired Auror. They would have a wealth of practical knowledge about magic and how to use it in dangerous situations. They could teach Harry how to sense dark magic and protect himself better.
Definitely Hermione. Her intelligence and vast knowledge of magic would be a great asset if cloned. She could help solve complex magical problems and come up with strategies in battles.
One interesting plot could be a tutor helping a Hogwarts student master a difficult subject like Ancient Runes in a very unorthodox way. Maybe the tutor has a unique magical ability that they use to make the learning process more engaging.
It could be Hermione. She's always been very book - smart and she could have decided to study law in the wizarding world. She already has a great understanding of the rules and regulations at Hogwarts, so it would be a natural progression for her to become a lawyer. And she would be very dedicated to helping Harry with any legal issues.
Voldemort's remaining Death Eaters could be the ones. Even after Voldemort's defeat, some of them might still be lurking in the shadows. They could force Harry into marriage as a form of revenge or to try and corrupt his good nature. They might think that by making him marry someone of their choosing, they can turn him to the dark side. Also, a group of radical pure - blood supremacists who see Harry as a threat to their traditional values might force him into marriage to try and subdue him and make him conform to their ideas.
Dumbledore could be an unexpected ally. He might see this as an opportunity for Harry to stand up for himself and gain some closure on his difficult childhood. Also, some of the Hogwarts professors, like McGonagall, could be on Harry's side. They have always cared for Harry's well - being and would not tolerate the Dursleys' abuse. And there could be some new magical characters introduced specifically to help Harry in his legal battle, like a wizard lawyer who has a soft spot for underdog cases.
Another possibility is Winky. She is also a house - elf. Winky was a bit of a tragic character in the original books. In the fanfiction, if Harry were to summon a servant, her meek and somewhat lost nature might make for an interesting addition to Harry's side, and she could be summoned to help Harry with various tasks.
There's a fanfiction called 'Snape's Guidance'. It shows Snape using his expertise in potions and dark arts to tutor Harry in a way that the Hogwarts curriculum doesn't cover. It also explores the emotional side of their relationship as Snape has to overcome his dislike for James Potter when dealing with Harry. 'A New Start with Snape' is also great. In this, Snape starts tutoring Harry after realizing Harry's potential to defeat Voldemort, and it has some really exciting plot twists as Harry discovers new magical abilities under Snape's tutelage.