One possibility could be that it's a result of some powerful magic gone awry. Maybe during a spellcasting class or an encounter with a mysterious magical artifact. This new development might lead to him being ostracized at first as others are afraid or jealous. But then he could use his angel wings to his advantage, like flying faster and more stealthily than on a broomstick.
Hermione would probably be both curious and concerned. She'd want to research immediately to find out how and why this happened. Ron, on the other hand, might be a bit more in awe at first. He'd likely make some jokes about Harry looking like a strange bird, but still be supportive.
Perhaps the wings grow as a result of some ancient magic that has been awakened within Harry. He could be exploring an old, forgotten part of Hogwarts and accidentally triggers a spell or a curse that gives him wings. At first, he might be scared and try to hide them, but as he learns to control them, he realizes they can be used for good, like helping him rescue trapped students or animals in the Forbidden Forest.
There could be many amateur authors out there who might write such a fanfiction. It could be a young, creative Harry Potter fan who loves to imagine new and exciting adventures for the characters. They might be inspired by their love for the series and the idea of giving Harry a new and unique ability like growing wings.
There could be a magical creature that takes on a human - like form and has black angel wings. This creature could be a guardian of some sort, maybe of an important magical artifact. It might initially be seen as a threat but is actually there to protect the wizarding world. When Harry discovers this creature, they form an unlikely bond, and the black angel wings become a symbol of their unique relationship.
I'm not sure of a specific 'Harry Potter' fanfiction just about angel wings off the top of my head. But you can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. You can search for the tags 'Harry Potter' and 'angel wings' and browse through the results. There are likely to be some creative and interesting stories there.
Hermione would probably be very curious at first. She'd immediately start looking through all her books to find any information about such a strange transformation. Ron, on the other hand, might be a bit more freaked out. He'd probably make some jokes at first to cover up his unease, but then he'd be there to support Harry. Ginny would likely be concerned but also find it kind of cool and would try to help Harry get used to his new wings.
One possible storyline could be that Harry discovers his angel wings during a dangerous battle at Hogwarts. The wings give him the ability to fly faster and more gracefully than a broomstick, allowing him to outmaneuver the dark forces. He could use them to rescue his friends from precarious situations, like flying them to safety when the castle is under siege.
One possible 'Harry Potter' fanfiction with angel wings could be about a character, perhaps Harry himself, suddenly sprouting angel wings during a dangerous encounter. It might be a magical transformation that gives him new powers to fight against dark forces. For example, the wings could allow him to fly faster than a broomstick and he could use this new ability to escape from Voldemort's clutches or to quickly reach his friends in need.
Maybe Harry is exploring an ancient and forgotten part of Hogwarts when he stumbles upon a magical artifact that causes him to build a cocoon. Once he grows wings from this cocoon, he finds that he has a connection to a long - lost race of magical beings with wings. He embarks on a journey to discover more about this race, all while trying to keep his new form a secret from his friends at Hogwarts who might be afraid of his transformation.