There could be a character like Tom, a Smeltings student who is very disciplined but curious about the magical world of Hogwarts. He might be the main character who drives the story forward as he experiences all the new things at Hogwarts.
One interesting plotline could be the culture clash between Smeltings and Hogwarts. Since Smeltings is known for its strict and perhaps more traditional ways compared to Hogwarts' more inclusive and magical diversity. For example, a Smeltings student might struggle to adapt to the use of magic for everyday tasks at Hogwarts, like using a levitation charm instead of doing things manually as they were taught at Smeltings.
Naruto would obviously be one of the main characters. And for the Hogwarts side, it could be Harry Potter himself. Imagine Naruto and Harry having a unique connection that develops into a yaoi relationship. They both have a sense of justice and a willingness to fight for what's right, which could be the basis for their relationship.
His family members also play roles. His father, Harry Potter, could be mentioned, perhaps giving Albus advice or being a source of inspiration. His siblings might also be included, adding family - related sub - plots to the story.
Maybe it could be a new character who is a combination of ninja and wizardry evils. This character could be jealous of Naruto's unique abilities that bridge the gap between two different worlds and wants to stop Naruto from uniting the ninja and wizarding communities against him. They could have powers that are a mix of dark magic and some dark ninja jutsus.
It could be a new dark wizard who has been lurking in the shadows. Maybe someone who was once a student at Hogwarts but turned to the dark side.
The main antagonists could be those who don't want Harry to receive a proper Hogwarts education. Maybe some dark wizards who fear that Harry will become too powerful if he goes through the Hogwarts training. So, they plot to keep him away from Hogwarts.
Sakura could have interactions with the Weasley family. Ginny Weasley might become her friend and they could have girl - talk about magic and boys. Fred and George could also be part of her story. They might play some pranks on her at first but then become good friends. And don't forget Neville Longbottom. Sakura could help Neville gain more confidence in his magic abilities as they study together in the Hogwarts library.
In this fanfiction, Dudley would surely interact with the Hogwarts professors. Professors like McGonagall would be strict with him, making sure he follows the rules. He might also cross paths with Draco Malfoy. Given Malfoy's attitude towards Muggle - borns, there could be some conflict there. And let's not forget the other students in his house. They would play a big role in his Hogwarts experience, either befriending or ostracizing him.
There could also be some new characters introduced. Maybe a student who also had a late start in a previous year and can give Harry some advice. Or a magical creature that Harry encounters on his way to Hogwarts late and that creature becomes some sort of guide or protector for him during his time at Hogwarts.
Maybe there's also an old Hogwarts professor who has some knowledge about the phantom's existence. This professor could play a crucial role in guiding the students or trying to solve the mystery related to the phantom.