Well, the plot could start with Harry having strange dreams or abilities that he can't explain with magic alone. As he investigates, he uncovers the truth about his mother being Supergirl. This revelation shakes his world. He then has to train to control his Kryptonian powers while still being involved in the wizarding world. There could be a conflict where the wizarding world sees his non - magical powers as a threat and tries to banish him, but he has to prove that he can use his powers for good and protect both the wizarding and non - wizarding worlds.
The characters in Harry Potter might be a bit shocked at first. Hermione would probably be very curious about Supergirl's powers and how they work. Ron might be a bit jealous at first but would eventually come around. Dumbledore, if he were still alive, would likely see it as an opportunity for the wizarding world to expand its horizons. In the Supergirl world, Kara's friends like Alex would be protective at first but would see Harry's good heart. J'onn J'onzz would be intrigued by the magic and how it could be used in their fight against evil.
A 'Harry Potter marries Supergirl' fanfiction might start with Supergirl accidentally landing in the wizarding world. Harry Potter, being the curious and brave wizard he is, approaches her. At first, there are misunderstandings due to their different cultures and powers. But as they start to communicate and learn about each other's worlds, they realize they have a lot in common. They both fight for the good of their respective worlds. This mutual understanding leads to a deep friendship and later, love. Their marriage would be a grand event, attended by wizards, witches, superheroes, and supervillains (the ones who are allowed to come under a truce). It would symbolize the coming together of two very different yet equally amazing fictional universes.
Well, it could be that Supergirl lands in the wizarding world somehow. Maybe she's chasing a bad guy who uses magic to escape to Hogwarts. And then she teams up with Harry Potter to fight the bad guy. Harry could teach her about magic while she teaches him about her superpowers.
One possible plot could be that fem Harry Potter somehow ends up in the Supergirl universe. She uses her magic to help Supergirl fight against a powerful villain who has a weakness to magic. Along the way, she has to adapt to the new world and its rules.
Well, it could be that Aragorn somehow time - travels or there's a magical mix - up that results in him being Harry's father. Maybe Harry would inherit some of Aragorn's leadership skills and courage in a more obvious way. He could use a sword like Aragorn's Andúril in battles against Voldemort.
Well, in a 'Harry Potter son of Aquaman' fanfiction, the story could start with Harry being an outcast at Hogwarts because of his strange connection to the ocean. As he grows older, he realizes he can control water in ways other wizards can't. This leads to him being both feared and admired. He could then go on a quest to find out more about his father Aquaman and in the process, use his combined magical and Aquaman - like powers to defeat a dark force that threatens both the wizarding world and the underwater kingdom.
The plot might involve Harry being hunted by forces that fear his newfound connection to Galactus. He could be seen as a threat not only to the dark wizards but also to the Ministry of Magic who don't understand his powers. Along the way, he could meet other characters from the Marvel universe who are either trying to help him or stop him, depending on their own interests. His friends, like Hermione and Ron, would have to struggle to keep up with his new abilities and the danger he attracts.
One possible plot could be that Harry, as the son of Broly, has an inborn, extreme power. He might struggle to control it at Hogwarts, much like he struggles with his normal wizarding abilities in the original series. His power could draw unwanted attention from dark wizards who either want to use him or destroy him out of fear.
Well, in such a fanfiction, Harry Potter might grow up with Supergirl's values. He could be more open about his magic in the world, as Supergirl is open about her powers. Maybe he'd be more fearless in facing Voldemort, having been influenced by Supergirl's courage.
Well, perhaps Harry would grow up with superpowers. Supergirl could teach him about using his powers for good. He might become a sidekick of sorts, fighting crime in their city. They could have a really cool dynamic where Harry is learning all these new things from Supergirl.