Hermione is always a great character to include. As a kid, she could be shown solving even more complex problems than in the books. Her intelligence and determination can drive the plot forward in fanfiction.
Obviously, Harry and Gabrielle themselves are essential. But characters like Hermione can add a lot too. She could be the one to help Gabrielle with her studies when she visits Hogwarts. Ron can also be there for some comic relief and to show the difference between the Hogwarts and Beauxbatons students.
Definitely Dumbledore. His wisdom would be crucial in guiding Harry and Maxima in any situation. Hermione would also be great as she could research and figure out how Maxima's powers work in relation to magic. And of course, Hagrid, who could introduce Maxima to the more unique and dangerous magical creatures in the wizarding world.
Neville Longbottom is a wonderful character to add. He has grown so much throughout the series and can be a great support to the trio. Dumbledore, even if in a ghost - like or memory form, can offer his great wisdom. Also, Draco Malfoy can be included in an interesting way. Maybe in fanfiction, he has a change of heart and teams up with the trio against a greater evil.
Obviously, Harry, James, and Lily should be central. But Sirius Black would be a great addition. He was James' best friend, so he could provide a lot of insights into their friendship and the time they spent together at Hogwarts. Also, Remus Lupin could be included as he was part of their group. He could add depth to the story by sharing his memories of James and Lily.
Luna Lovegood would be an interesting character to include. Her unique perspective on magic could help Harry and Molly in unexpected ways. For example, she might know of some strange magical creatures that are relevant to a mystery they are trying to solve. Hagrid is another great choice. His love for magical creatures and his big heart could bring a lot to the story. He could help Molly and Harry take care of some new and dangerous magical beasts that show up in the fanfiction.
Sherlock Holmes is a great choice. His deductive skills would contrast well with Kaito Kid's thieving skills. Another good one is Lupin III. They are both master thieves, and a crossover between them could be full of interesting heist scenarios. Conan Edogawa would also be interesting. Since he's always trying to catch Kaito Kid in the original series, a crossover could explore new dynamics between them.
Sally Jackson, Percy's mother, is also an important character. Her love for Percy and her strength can be a central part of the story. She could be the one who gives Percy the emotional support he needs when he's confused or scared about the strange things happening around him. In a fanfiction, she could also have some secrets of her own related to the mythological world that Percy discovers as he grows up.
Definitely Sasuke. His relationship with Naruto is so important in the original story, and when it comes to a fanfiction with kid Naruto and Boruto, Sasuke can act as a mentor or a challenging rival for both of them. Another great character is Hinata. She has a special connection to Naruto, and in a fanfiction, she could play a role in guiding Boruto or providing support to kid Naruto in a new situation.
Dumbledore would be an interesting addition. He has a lot of knowledge and power, and his interaction with Merlin would be fascinating. Ron could also be included for some comic relief and his loyalty would be an asset in any adventure that combines the two universes.
Well, Sirius Black could be an interesting character to put in a 'Harry Potter Dursleys fanfic'. Since he was Harry's godfather, his presence could stir things up for the Dursleys. He might be furious at how they treated Harry and take some sort of action. Hermione and Ron could also be included. They could visit Harry at the Dursleys' and be shocked at the living conditions. Their reactions and how they interact with the Dursleys could make for an exciting plot. And of course, the Weasley family as a whole. Their warm and loving nature would contrast greatly with the cold Dursleys.