Dumbledore could be involved too. As the headmaster, he has a lot of knowledge about the vaults at Gringotts. He might offer some cryptic advice to Harry about Vault 99. Also, the Weasley twins could make an appearance. They're known for their mischief and might be able to help Harry get into places he otherwise couldn't, like the vault or places where Malfoy is hiding secrets related to it.
Neville could play a part. He's grown a lot in courage throughout the series. He could be a valuable ally to Harry. And Snape might be involved in a more complex way. Maybe he has some knowledge about Malfoy's activities related to Vault 99 due to his spying for Dumbledore. His actions could either help or hinder Harry's quest to uncover the truth about the vault.
Hermione would probably be involved. She's always by Harry's side when it comes to solving mysteries. She could use her vast knowledge to help Harry figure out what's so special about Vault 99. Ron might also be there. He's Harry's loyal friend and would be ready to back him up in any adventure related to Malfoy and the vault.
Hermione would probably be involved. She's always by Harry's side, and she might be researching ways to help Harry tame the basilisk. Maybe she finds some useful spells or information in the library that gives Harry an edge.
Hermione could be involved. She might suspect Daphne of having ulterior motives regarding Harry and try to warn him, which could play into the betrayal storyline in some way, either as a catalyst or as someone who helps Harry deal with the betrayal later.
Definitely Hermione and Ron. Hermione would use her intelligence to analyze the situation and come up with a plan. Ron would be the more impulsive one, ready to jump into action. Sirius Black could also be involved. Since he has a history with Pettigrew, he might have some insights into Pettigrew's actions and be able to help in the search for Harry. Also, the Weasley family in general might get involved as they are close to Harry.
The plot could center around the mystery of Vault 99. When Harry finds out Malfoy has access to it, he starts investigating. He might discover that the vault holds some sort of power or knowledge that could change the wizarding world. Harry would then be in a battle of wits with Malfoy to either gain control of the vault's contents or prevent Malfoy from misusing them. This could involve sneaking around Hogwarts, using magic to uncover clues, and facing off against Malfoy's allies.
Hermione would definitely be involved. She's always been by Harry's side, and in this kind of fanfic, she would likely be helping Harry in his struggle, using her intelligence to figure out solutions to the problems he faces. Ron would also be there, providing his loyalty and courage in the fight. Dumbledore might make an appearance too, perhaps as a guiding spirit or in flashbacks, offering his wisdom to Harry as he battles towards salvation.
There could be some original Muggle characters created just for the fanfiction. Maybe a friendly Muggle neighbor who is always curious about Draco's strange ways and in turn, makes Draco curious about the Muggle world. This neighbor could introduce Draco to things like Muggle sports, like football, which Draco would find both strange and exciting.
Hermione is a must. Her intelligence and extensive knowledge of magic and Hogwarts rules would be invaluable. Ron would be involved too. He's Harry's close friend and has a bit of a mischievous streak himself. Fred and George Weasley are very likely to be part of it. They are known for their love of pranks and their dislike of authority figures like Umbridge. Luna Lovegood could also be included. She has a unique perspective and might come up with some really out - of - the - box ideas for Harry's revenge on Umbridge. Plus, her presence would add an interesting and somewhat dreamy element to the whole revenge plot.
Definitely the Weasley family. Molly and Arthur Weasley would be very much involved. Molly would be all excited about the wedding preparations, fussing over Ginny's dress and the food for the reception. Arthur would be there to offer his support and probably share some interesting wizarding stories. Ron and Hermione would also be there. Ron as Harry's best friend would be part of the pre - wedding jitters and celebrations. Hermione would be helping Ginny with all the details, like choosing the right flowers and making sure the wedding venue is decorated perfectly.
There could be his drill sergeant, a tough and no - nonsense character who has to whip Harry into shape. Maybe there's a fellow recruit who becomes Harry's best friend. They could support each other through the difficult training.
There could be his friends like Ron and Hermione. Even though he hates everyone, they would still be important characters as they might try to figure out what's wrong with Harry and how to bring him back from this state of hating everyone.