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.
One great Hermione, Bill, Charlie, Fred, and George fanfiction could be a story where they all team up for a dangerous magical quest. Maybe they have to find a lost artifact that is crucial for the wizarding world. Hermione's intelligence would lead the way in researching about the artifact, while the Weasley brothers - Bill with his worldly experience, Charlie with his dragon knowledge, and Fred and George with their creativity and mischievousness - would contribute in different ways like handling the physical challenges along the way.
One common theme is friendship. Many fanfictions focus on how Hermione can form deep and meaningful friendships with Bill and Charlie. Another theme is adventure. Given Bill's work with Gringotts and Charlie's with dragons, they often embark on exciting adventures with Hermione in these stories.
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.
Overcoming differences is also a theme. Since they are two distinct characters, they may have different personalities or beliefs. Fanfictions often show how they learn to accept and work with each other's differences, which can be really inspiring for readers.
One more theme is growth. In these fanfictions, we can see Harry, Charlie and Bill grow as individuals. They learn from their experiences, become more powerful in magic, and develop better understanding of themselves and others. This growth is often tied to the challenges they face together and the relationships they build.
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.
Yes, there are quite a few out there. You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for the specific tag or keywords related to 'hermione is bill and charlie's mate' and you'll likely find some interesting stories.