Harry Potter could use his money to improve Hogwarts in fanfiction. He might donate to renovate old classrooms, buy new equipment for the library or even fund extra - curricular activities for students. After all, he has a strong connection to the school and wants to give back.
He might meet some old retired wizards who can tell him stories about the old days of magic. For example, they could share tales of how Hogwarts was founded or the great wizarding wars long ago.
Maybe he needs the money to fund an independent research project. He's always been curious about certain magical phenomena, and withdrawing his money would give him the financial means to pursue that knowledge without relying on Hogwarts or the Ministry.
He might use it to buy rare magical artifacts. For example, he could purchase ancient wands or enchanted jewelry that have special powers. This not only adds to his personal collection but also gives him an edge in certain situations.
One significant event could be when Harry decides to leave Hogwarts temporarily to train with a reclusive but powerful wizard. This step shows his determination to take control of his magical growth and not be confined to the standard Hogwarts education. It's a big decision as it goes against the norm and he risks a lot, but it also gives him the opportunity to learn things that can help him better face Voldemort or other threats.
One great 'Harry Potter Takes Control' fanfiction is 'The Power Within Harry' which shows Harry taking control of his destiny in a unique way. It has him making decisions that are different from the original story, like taking charge of his magical training earlier.
Well, it could be about Harry Potter having a kind heart and taking in various 'strays' in the wizarding world. Maybe he takes in lonely or lost magical creatures, or perhaps other students who don't really have a place to belong. It might explore his nurturing side and how these 'strays' change his life or the Hogwarts environment.
Perhaps he leaves Gryffindor in fanfiction because he is influenced by a new mentor or friend who is from a different house or group. This person might convince him that he can achieve more or be more true to himself outside of Gryffindor. Also, it could be due to a prophecy or a special mission that requires him to distance himself from Gryffindor for a while to protect the house or the entire Hogwarts from a certain threat.
In this fanfiction, it could be that Harry takes over the House of Potter to restore its former glory. He might have to deal with dark forces that have been lurking around the property, and he uses his skills and the help of his friends to cleanse it. Maybe he uncovers a plot against the wizarding world that was somehow connected to his family's house, and he has to prevent it from coming to fruition.