Maybe the older brother was separated from Harry when they were young. He could have been kidnapped by Death Eaters who thought he had some special power. Years later, he returns, having been trained in dark magic but still having a good heart deep down. He has to struggle with the dark influence while also trying to reconnect with Harry and their family at Hogwarts.
One possible plotline could be that the older brother is a prodigy at magic and overshadows Harry in the beginning. For example, he might be the top of his class in every subject at Hogwarts. This could create some jealousy or a sense of inadequacy in Harry at first. But as the story progresses, they could learn to support each other and the older brother could use his skills to help Harry in his battles against Voldemort.
The older brother could be a Squib in this fanfiction. While Harry is a famous wizard, his older brother has to deal with the disappointment of not having magic. This could lead to a story of the older brother finding his own way in the non - magical world but still being an important part of Harry's life, perhaps giving him advice from a different perspective on how to handle the Muggle - wizard relations.
One possible plotline could be that Harry discovers he has a half - brother he never knew about. Maybe his father had a relationship before he met Harry's mother, and this new character enters Harry's life. They initially have some conflicts due to jealousy or differences in upbringing, but over time, they come to love and support each other.
One possible plotline could be that they discover their relation later in life. Harry, being used to the magical world, is shocked to find out he has a brother in a completely different scientific realm. And Bruce has to come to terms with having a wizard for a brother. They then start to learn from each other's worlds, with Harry showing Bruce some basic magic and Bruce trying to explain his scientific research to Harry.
One plotline could be that they discover their relation later in life. Harry, having grown up in the wizarding world, and Superman, in his superhero world, suddenly find out they are brothers. They then have to combine their powers and knowledge to face a common enemy that threatens both their worlds, like a powerful dark wizard who has also got hold of some kryptonite - like substance.
One plotline could be Naruto being a transfer student at Hogwarts. He might struggle to adapt to the magic system while also showing off his ninja skills. Another could be Naruto teaming up with Harry Potter to fight a common enemy that threatens both the ninja world and the wizarding world.
One possible plotline could be Harry using his knowledge of the wizarding laws, which he learned through his years at Hogwarts and his experiences with the Ministry of Magic, to defend wrongly accused wizards in a wizarding court. Another could be him trying to reform the wizarding legal system which he might find full of loopholes and biases.
One possible plotline could be Harry trying to hide his merman identity while at Hogwarts. He might face challenges in the magical underwater areas of the school, like the lake. Another could be that he discovers his merman heritage and has to go on a journey to learn more about it and his new powers.
One possible plotline could be Harry being secretly trained by an unknown figure to become an assassin against the dark forces. He would have to hide this new identity from his friends at Hogwarts.
One possible plotline could be about the Weasley family trying to improve their financial situation. Maybe they come up with a new business idea in the wizarding world, like a magical product that becomes a huge success and changes their economic status. Another could be a story about how the Ministry of Magic regulates the financial system in the wizarding world, and there are some characters who try to manipulate or reform it.
In some fanfictions, the Queen might be a long - lost relative of a pure - blood wizarding family. This would lead to her being involved in the complex politics of the wizarding world. She could use her position as the Queen to try and bring about change in the wizarding world's old - fashioned pure - blood - supremacy - based society.
One possible plotline could be a Lannister student at Hogwarts. They might bring their family's ambition and power - hungry nature into the school, causing all sorts of drama among the houses. For example, trying to take over the Slytherin house leadership through manipulation.