In some cases, it could be a corrupt Ministry of Magic official. This official might be trying to cover up some dark secrets related to magical creatures or certain magical areas that Bill, Charlie, and Hermione are investigating. The official would use his power and influence to try to stop them from uncovering the truth, creating a conflict where the three protagonists have to fight against the system to do what is right.
It could be a dark wizard who has a vendetta against the Weasley family. Since Harry is with Bill and Charlie, he gets caught up in the conflict. This dark wizard might see Harry as an easy target because of his shyness, thinking he can be easily intimidated.
One possible plot could be that Hermione meets Bill and Charlie during a magical creature research project. Bill's extensive knowledge of magical creatures from his work with Gringotts and Charlie's expertise from his time at the dragon reserve in Romania could complement Hermione's academic approach. They might start off a bit skeptical of each other, but as they work together, they come to respect and rely on one another, with Hermione learning a great deal from their practical experiences.
Well, perhaps they could go on an adventure together. Maybe they discover a hidden magical place in the Forbidden Forest. They might use their combined skills, Harry's courage, Bill's experience with magical creatures, and Charlie's knowledge of dragons to overcome some sort of danger or mystery.
In this fanfiction, perhaps Harry will be initially hesitant and quiet when meeting Bill and Charlie. He might be in awe of their more experienced and confident personas. Bill could offer Harry some words of wisdom regarding the wizarding world outside of Hogwarts. Charlie, on the other hand, might share stories about dragons which would both fascinate and intimidate Harry.
They could help Harry by teaching him new spells. Bill might teach him some advanced defensive spells he learned as a curse - breaker. Charlie could show Harry some unique ways to use magic around magical creatures. Hermione would of course be there to ensure Harry studies hard and understands the theory behind the spells.
In a fanfiction scenario, perhaps Hermione has a research project and enlists the help of Bill, Charlie, and Harry. Bill could offer insights from his travels, Charlie might contribute with his expertise on dragons as it relates to the research, and Harry would be there for moral support and to offer his unique perspective as the Boy Who Lived. It could be a story of friendship and cooperation in the magical community.
They could be like brothers. Harry has a strong connection with the Weasley family already, and with Bill and Charlie, it could be a natural extension of that. They might look out for each other and support one another in any difficult situation.
Another possibility is a dark wizard or a group of dark wizards who are after some powerful magic that the Lestrange family has, and Hermione gets in their way as she tries to protect the family or uncover the secrets. They would be the main force that Hermione has to face and overcome throughout the fanfiction.
Perhaps a powerful magical being from a different realm is the antagonist. This being might mistake Hermione for someone else or see her as a key to something in the wizarding world. So, it uses its magic to make her disappear. This being would be very difficult to defeat or negotiate with, making it a huge challenge for those trying to find Hermione.
The plot might involve Hermione going on an exchange program to study magical creatures in a different country where Charlie is based. They meet and Charlie shows Hermione around the dragon habitats. As they spend time together, they discover they have a shared love for protecting magical creatures and a mutual respect for each other's knowledge. Eventually, they become friends and help each other out in various ways, like Hermione helping Charlie with some paperwork using her organizational skills.