A really cool MHA Harry Potter crossover fanfiction is 'Heroes at Hogwarts'. In this story, the unique powers of the MHA characters interact in fascinating ways with the magic of Harry Potter. For example, Deku's One for All quirk has some unexpected interactions with spells. It's a must - read for fans of both series.
He could be shocked. Maybe there are some fanfictions that take a very dark turn or have unexpected pairings that he never thought of. He'd probably need some time to process such strange ideas. And he might also be curious to see how far the creativity of the fans can go. He might even get some new ideas about his own life from reading some of the more imaginative fanfictions.
A villain like Shigaraki would likely be interested in the power levels in the MCU fanfiction. He's always looking for ways to increase his own power, and seeing the various powerful artifacts and abilities in the MCU might give him some ideas. He could think about how he could use similar things in his own plans to take over the MHA world.
Another option is Reddit. There are subreddits dedicated to fanfiction, like r/FanFiction. You can post a request for MHA Harry Potter fanfiction there, and the community might be able to recommend some great reads. Also, sometimes people share their own works on these subreddits.
The magic system could be incorporated by having Hogwarts - like schools in the MHA universe. Teachers could teach spells and potions, and students would develop their magical abilities as they do their quirks. Magic could also be used in combat situations in the MHA world. For instance, a character could use a levitation spell to gain an advantage in a fight against a villain. This would add a new layer of strategy to battles in the MHA world.
A cool plot might be that the Hogwarts students attend U.A. High School. Hermione would surely excel in academic subjects there, while Ron could struggle a bit more. And Harry could use his experience with fighting dark magic to help in hero training. They could also introduce magic - based quirks to the MHA world, which would change the dynamic of how heroics are done.
One interesting crossover could be the magic system of Harry Potter being introduced to the MHA world. For example, a character from MHA might discover they have magical abilities like a Hogwarts student. Another could be the heroes from MHA having to face off against dark wizards from the Harry Potter universe. It would be a really cool mix of the two different power dynamics.
He may be confused at first. Fanfics can be very different from his real experiences. He might wonder why people write such things about him and his world.
Well, Harry Potter is a rather humble character. He might initially feel a bit embarrassed to see himself as the center of so many fictional stories created by fans. If the fanfic portrays him as a hero in new adventures, he would probably be honored. However, if it misrepresents his friends or the values he holds dear, he could be a bit indignant. He would likely approach it with an open mind but also with a sense of protectiveness towards his own story and those he loves.
Harry might initially be shocked and confused. He's used to Hogwarts being his safe haven, so when it turns against him, he'll be at a loss. He may try to prove his innocence by finding evidence or confronting those who are spreading lies.