Hermione would be a great addition. She could help Harry and Lupin with research, especially when it comes to matters related to werewolves or other magical mysteries. Her intelligence and quick thinking would be valuable assets in the story.
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.
Definitely Harry Potter. He was a good friend of Lupin and Tonks, so he could be like an uncle figure to the baby. He could visit often and share his own experiences growing up in the wizarding world, giving the baby some valuable advice.
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.
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.
Definitely Hermione. She's so intelligent and can offer a different perspective. She could help Teddy with his studies and give him advice on how to deal with the academic pressure at Hogwarts. Also, Ron. His sense of humor can lighten the mood. He could be like an older brother to Teddy, joking around with him and teaching him some fun wizarding tricks.
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.
There are also authors who are really good at character development in their fanfictions. Take 'TonksTales' for example. They create stories where Lupin and Tonks' characters are very true to the original books but also have new and interesting facets added. They might explore Lupin's insecurities in a new light or show Tonks' Metamorphmagus abilities in unique situations. Another great author could be 'RemusAndNymphadora' who writes stories that are often set in different time periods of the 'Harry Potter' world related to Lupin and Tonks.
Adding Draco Malfoy could create some interesting drama. He could use Harry and Ginny's jealous games to his advantage, maybe trying to drive a wedge between them. And of course, the Weasley family as a whole. Molly Weasley could notice Ginny's behavior and have a motherly talk with her, while the other Weasley siblings could have different reactions to the whole jealous situation between Harry and Ginny.