The story could develop in a very different way. Harry might continue to live a normal Muggle life, completely unaware of his magical abilities. He would probably grow up thinking he was just an ordinary boy, going to a normal school and having normal friends.
In the fanfiction, it could be that Harry's aunt and uncle managed to intercept the letter before it got to Harry. They were so determined to keep Harry away from the wizarding world that they might have found a way to stop the owl or trick the postal system. They could have used some kind of Muggle - magic repelling charm they got hold of somehow. This would be a very interesting twist in the story as it shows how far they would go to keep Harry ordinary.
In some fanfictions, when Harry Potter gets his letter at 9, it might be a moment of great excitement. It could be the start of his journey into the wizarding world just like in the original story, but with some unique twists added by the fanfiction author. Maybe the owl that delivers the letter has some special markings or behaves in an unexpected way.
Maybe it's a magical protection. In the wizarding world, there could be some ancient magic associated with hair length. Long hair might act as a sort of shield or a connection to some deeper magic that Harry unknowingly benefits from.
Dear [Character Name], This letter of thanks is long overdue. You've been an amazing part of my Hogwarts journey. Whether it was in Potions class, where you helped me avoid blowing up my cauldron, or on the Quidditch pitch, cheering me on from the stands. Your kindness and selflessness have not gone unnoticed. I've learned so much from you, from how to handle a difficult Boggart to how to find the beauty in the simplest of magic spells. You are truly a gem in the wizarding world, and I am so lucky to have you in my life. Thank you with all my heart. [Your Name]
The story might progress with Harry becoming bitter about never getting the letter. He could start to resent the wizarding world. However, as he grows older, he might start to notice strange things happening around him that he can't explain. Eventually, he could start to investigate on his own and find out about Hogwarts and the wizarding world in a more roundabout way. This could lead to him having a more complex relationship with the magical community compared to the original story.
One possible reason could be that the owl got lost on its way. Maybe it was attacked by some magical creature or got distracted by a powerful spell. Another reason might be that someone deliberately intercepted the letter. In the wizarding world, there could be dark forces at work who don't want Harry to go to Hogwarts. It could also be a bureaucratic mix - up at Hogwarts itself. Maybe the admissions office made an error in their records and didn't send the letter.
I'm not sure specifically as I haven't read that exact fanfiction. But generally, it might be about a castle in the Harry Potter universe that doesn't exist in the original books. It could be a place full of mystery, perhaps with its own unique magic, and the fanfiction might explore the adventures of the characters in this never - was castle.
Some might find it charming. For example, his friends like Ron and Hermione might see it as just another part of Harry's unique personality. It could make him seem more approachable to those who are drawn to his non - conformist ways.
A Hogwarts student, like Ron. Ron could hand - deliver a letter from his mother to Harry. Mrs. Weasley might be worried about Harry and send a letter full of motherly advice and a knitted jumper. Ron's delivery would add a touch of friendship and family - like bond between the Weasleys and Harry.