Seamus Finnigan could be an interesting character to put in. His sometimes - explosive magic could create some comical and dangerous situations for the group. And of course, Hagrid. He could introduce them to all kinds of magical creatures and give them advice from his own experiences at Hogwarts.
I think Ginny Weasley would be a good fit. She's strong - willed and has her own share of magical abilities. She could be a great friend to Harry and Neville, and together they could form a sort of mini - Dumbledore's Army, going on secret missions around Hogwarts to protect the school and its students.
Luna Lovegood could also be an interesting addition. Her unique perspective on magic and the wizarding world might bring new ideas to the group. For instance, she could have insights about some of the more mysterious magical creatures or phenomena that they come across, which could be crucial in their quests or battles.
Akeno Himejima could also be a great addition. Her dual nature of being sweet but also having a darker side would create an interesting dynamic with Harry. They could have some really engaging conversations about power and morality.
Harry Potter is likely to be one of the main characters. He and Neville often interact in the original series, and in the fanfic, their relationship could be further developed. Hermione Granger could also be a main character. Her intelligence and knowledge of magic could play a role in whatever adventures or situations Neville gets into. Ron Weasley might be there too, adding his humor and loyalty to the group.
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.
Hermione would be a great addition. She could provide her vast knowledge of magic and books to help Harry and Krum in their adventures. For example, she might research spells or potions that are needed to overcome certain obstacles.
One great fanfiction could be 'The Unlikely Bond'. In this story, Harry and Neville's friendship grows as they face new challenges together in the Forbidden Forest. They discover a hidden magical creature and have to protect it from dark wizards. It shows how their different personalities complement each other.
Their friendship could lead to some interesting team - ups in Quidditch. Neville might not be the best flyer, but he could be the strategist for Harry. Harry, on the other hand, could execute Neville's plans on the broomstick. This would not only strengthen their bond but also show how their different skills can complement each other.
Maybe a fanfic about how Harry and Neville become best friends during a particularly difficult year at Hogwarts. They support each other through exams, Quidditch matches, and dealing with bullies like Draco Malfoy. Neville could help Harry with his Herbology studies, and Harry could teach Neville some useful Defense Against the Dark Arts spells.
Obviously, Harry Potter and Nicolas Flamel are essential. But also including characters like Hermione Granger would be great. Hermione's thirst for knowledge would make her a perfect student of Flamel's teachings.