Even some minor characters could be involved. For instance, Neville Longbottom could accidentally stumble upon a way to help Harry while looking for something else. Or a magical creature like Fawkes, Dumbledore's phoenix, could show up and use its powers to free Harry. There could also be a new, original character created by the fanfiction author who has a special ability or knowledge to assist Harry in his escape.
Dumbledore might play a role. He could be the one who encourages Harry to stand up to the Dursleys. He might have some hidden knowledge about the relationship between the wizarding world and the Dursleys that could help Harry. Also, Hagrid could be involved. He could provide some muscle if things get physical. He might also offer some emotional support to Harry, as he's always been like a big, friendly giant to him.
Dumbledore could also play a role, even if it's from beyond the grave. Maybe he left behind some clues or instructions that Harry only discovers when he's in the middle of trying to remove the Horcrux.
Dumbledore could rescue Harry. He's always been watching over Harry and would surely sense if the Dursleys had abandoned him. Dumbledore has the power and wisdom to take Harry to a safe place, perhaps a different magical family or a special hideout. He could also start teaching Harry about his past and his parents earlier, which would give Harry a better understanding of his place in the wizarding world.
Petunia Dursley is also quite popular. There are fanfictions that delve into her past, her relationship with Lily, and how that affects her treatment of Harry. Some stories might show her having a change of heart and trying to make amends with Harry in a more complex way than just the basic storyline in the original books.
Hermione could be one to rescue Harry. She might have discovered some new spell or potion that allows her to bypass the protections around the Dursleys' house and get to Harry. Then she'd use her intelligence and quick thinking to get him out of there safely.
Common characters he could meet include an au Ginny who is a powerful seer. There might be a Hagrid who is a tiny, timid man instead of the large, boisterous one we know. He could also encounter an au Sirius Black who is a Death Eater, which would be a shocking twist. Additionally, he may run into an au Professor McGonagall who is more of an artist than a Transfiguration teacher.
Well, in such fanfiction, Harry Potter could meet different versions of Dumbledore. In one universe, Dumbledore could be a dark wizard instead of the wise mentor we know. He may also come across alternate versions of Sirius Black. Maybe in some universes, Sirius is still in Azkaban for a crime he actually committed. Additionally, he could meet different versions of Luna Lovegood, some of whom might have no interest in the strange and wonderful things she loves in the original universe.
Well, Harry Potter is surely one of the main characters since the title implies it's about him being taken from the Dursleys. There might also be some original characters created by the fanfiction writer to play important roles in Harry's new journey.
There could be a group of magical outcasts in Canada who take Harry in. These are people who have been shunned by the mainstream wizarding society for various reasons. They would understand Harry's need to escape and would protect him because they see him as one of their own, someone who is also trying to find a place where he can be safe and free.
He might meet versions of himself from different timelines or parallel universes. Also, there could be new and unique magical beings that the author creates just for the fanfiction. For example, a race of half - human, half - phoenix creatures.