Dumbledore could also be an ally. He is very knowledgeable and might give Hermione some advice or information about the vanishing cabinet that she needs to successfully sabotage it. He might also provide some protection if there are any risks associated with her actions.
The house - elves could potentially be her allies. They know a lot about the secrets of Hogwarts, including things about the vanishing cabinets. They could help Hermione by providing her with access to places she needs to be to carry out her sabotage, or by giving her some magical items or substances that can help in the process.
In the fanfiction, perhaps Hermione realizes that the vanishing cabinet is being misused by some students for pranks that are getting out of hand. These pranks could be endangering other students. So, she decides to sabotage it. She might use a spell to jam the mechanism inside the cabinet, making it impossible for things to vanish or appear through it.
Harry and Ron would be her most likely allies. They are her best friends, and even if they might be initially shocked or jealous of her being in the tournament, they would surely come around to support her. Another likely ally could be Cedric Diggory. He's a fair - minded and kind wizard, and he might see Hermione's intelligence and skills as an asset and offer to help or at least be friendly towards her.
Draco Malfoy is very common. Since he was the one who actually used the vanishing cabinet in the original books, a lot of fanfiction focuses on his motives and actions related to it. Harry Potter himself is also often in these stories. He might be trying to stop Draco from using the cabinet for evil purposes or exploring the secrets of the cabinet himself. Hermione Granger could be another one. Her intelligence might be used to figure out how the cabinet really works or to counter any dark magic associated with it.
Harry Potter himself is often a main character. Since the vanishing cabinet was an important object in the original series, fanfiction writers like to explore more of his adventures related to it. He might be the one trying to figure out the secrets of the cabinet or using it to escape danger.
Neville could be an ally too. His skills in herbology could come in handy when it comes to finding the right plants to feed the dragons or to use for medicinal purposes. Also, Luna Lovegood might be involved. Her unique perspective and knowledge of strange magical creatures could give Hermione new ideas on how to care for the dragons. She might know of some obscure spells or methods that are beneficial.
One interesting plot could be the discovery of new magical properties of the vanishing cabinet. For example, it might lead to a secret chamber in Hogwarts that has been long forgotten. Maybe it's filled with ancient spells and artifacts.
Harry and Ron could be her main allies. They've always been by her side in the original story, so it's likely they'd support her new adventure as a dragon rider. They could help her keep her dragon - riding activities a secret from the teachers at Hogwarts.
One idea could be a story where a character discovers a hidden power within the vanishing cabinet. For example, it might be able to not just transport people between Hogwarts and Borgin and Burkes, but also to other secret magical locations. Another could be a story where a group of students use the vanishing cabinet to time - travel in a way, perhaps going back to the days when the founders of Hogwarts were alive and experiencing their magic first - hand. Maybe a fanfiction could explore what would happen if the vanishing cabinet was broken and then repaired in a different way, causing it to have new and unexpected functions.
Her parents, of course. They would be an important part of the story as they react to the news. They might be shocked at first but then supportive.
Professor Snape might also be involved. Despite his usual strictness with Hermione, he is a potions master. He could see this as an opportunity to teach her a lesson about being more careful, but also help her because he wouldn't want a student to be in real danger from a botched potion. And there's always the possibility that Dumbledore, with his vast wisdom and knowledge of magic, could offer some cryptic advice that ultimately helps Hermione solve the problem caused by the accident.