Well, he might use his magic to solve Westeros' political problems. For example, he could use a spell to make the warring factions come to a peaceful negotiation. He might also introduce magic to the people of Westeros, which would completely change their way of life. And his skills in fighting dark forces could be useful in dealing with any threats in Westeros, like the White Walkers.
The knights in Westeros would be both intrigued and a bit intimidated. Harry's magic is something they've never seen before. Some might want to learn from him, while others might see him as a threat to their traditional way of fighting. The maesters would be very interested in studying his magic, trying to figure out how it works and how it could be incorporated into their knowledge. The children in Westeros would probably be in awe of him, especially if he shows off some cool magic tricks. They might start dreaming of becoming wizards themselves, which would be a huge change in their aspirations.
His friends like Ron and Hermione would surely support him. They have been through a lot together, and they believe in Harry's leadership. Also, the Weasley family in general would be behind him. They have always been on Harry's side in his battles against the dark forces.
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.
If Harry takes over the Ministry in the fanfiction, he could use his experience to better protect the wizarding world. He has faced Voldemort, so he knows the importance of unity. He would probably encourage more cooperation between different wizarding communities. He could also use his popularity to inspire young wizards and witches to be more involved in making the wizarding world a better place, like starting volunteer programs within the Ministry.
Well, he might start by changing all the unfair rules she had imposed at Hogwarts. He could bring back the fun and freedom in the school, like allowing students to form their own clubs again.
Well, he might start by reforming the House system. Maybe he'd make it more inclusive and less competitive. He could also introduce new classes that focus on practical magic for real - world situations.
In such a fanfiction, perhaps Harry Potter could use his magic to bring about major political changes across the world. He might establish a new global wizarding order where magic and Muggle affairs are more intertwined. For example, he could introduce magical solutions to global problems like climate change, using spells to clean the air or water.
There could be some antagonists as well. Maybe some dark wizards who were enemies of the Potter family in the past and are still lurking around the house, trying to stop Harry from taking full control. There might also be some Ministry of Magic officials who are interested in the House of Potter for various reasons, such as the possible presence of powerful magic items. And Neville Longbottom could also be involved, as he was a key figure in the fight against Voldemort and could be there to support Harry in this new adventure.
Some characters in Westeros might be very suspicious of Harry. His strange clothes and the way he does magic would make them think he's some sort of dark sorcerer. Others, like the more open - minded ones, might be curious and want to learn from him. For example, Tyrion Lannister might see Harry as an interesting oddity and try to figure out how his magic works.