One possible fanfiction could involve Nicholas Flamel using his alchemical knowledge to combat a plague that has spread in the wizarding world while Hermione assists him. Maybe she discovers ancient spells in the library that Flamel can incorporate into his potions to find a cure.
One common theme is the power of the Philosopher's Stone. Since Flamel is associated with it, many fanfictions explore how the Stone's power can be used or misused. For example, some stories might have villains trying to steal it again for their own dark purposes.
The plot could revolve around Harry's apprenticeship under Nicholas Flamel. Harry might be chosen because of his unique connection to Voldemort. Flamel could see that Harry has a special destiny and decides to train him in the ways of alchemy. This could include learning how to transmute substances, understand the properties of magical elements, and use alchemy for defense and offense. Harry could also learn about the history of magic through Flamel's eyes, which is much longer and deeper than what he has learned at Hogwarts. There could be a lot of adventures where Harry uses his newfound alchemical knowledge to fight against the forces of evil.
Perhaps Nicholas is a local in Manhattan who has a special Christmas every year. He could be someone with a big heart, always spreading Christmas cheer around the neighborhood. He might be a shopkeeper who gives out freebies to kids during the holiday season, or a musician who plays Christmas carols on the street corners.
I'm not entirely sure which one was her absolute first novel ever, but in terms of her popular Bishop - related series, it could be 'Stealing Shadows'. In this book, you get to meet characters who are part of the world she would expand upon in later novels. It has a great blend of mystery and a touch of the supernatural, which is characteristic of Kay Hooper's writing. It's a good starting point for those interested in exploring her works.
One possible plot could be that Harry grows up learning ancient alchemy from Flamel. He might be trained in the secrets of the Philosopher's Stone from a very young age, giving him a different perspective on magic compared to his canon self. Maybe he'd have a deeper understanding of life - extension magic and use it to help others in the wizarding world.
Many fanfics focus on Flamel's role in the wizarding world's history. They might show how his actions in the past have shaped the present situation in the wizarding world that Harry finds himself in. Some fanfics also have a plot point where Flamel's power is slowly fading, and Harry has to find a way to either restore it or use the remaining power to defeat Voldemort. There could also be a plot where Flamel has a prophetic vision about Harry and his destiny, and the story then revolves around how Harry tries to fulfill that destiny with Flamel's guidance.