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.
I think Neville Longbottom should be included. He has grown so much throughout the series and his courage and knowledge of plants could be an asset. Also, Remus Lupin. His experience as a werewolf and his teaching skills could come in handy. Maxima herself would bring her own set of friends or allies from her world, which would add more depth and variety to the fanfiction characters.
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.
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.
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, 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.
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.
Definitely Sirius Black. Since he was one of Lupin's best friends and also had a strong connection with Harry. His presence can add a lot of drama and backstory to the fanfiction.
Well, Hermione would be a great addition. She could use her intelligence to help Harry and Moka understand the different aspects of their two worlds. For example, she could research about yokai and find ways to use magic to help Moka control her vampire side better.
The Cullens from the Twilight series are obvious choices. They could interact with Harry in various ways. For example, Carlisle's medical knowledge and his calm, father - like presence could provide stability in a dangerous situation that Harry and Edward find themselves in.