The main characters would probably be the descendants of the famous wizarding families. They might go to the Dursleys' place for some reason, like to research the history of how Muggles and wizards interacted in the past. The Dursleys, being the typical Muggle family with their own set of values, would be the counterparts for these young magical characters. Their clashes and reconciliations could form the core of the story.
Most likely, the main characters from the next generation of the wizarding world, like the children of Harry Potter and his friends. And of course, the Dursleys.
Well, perhaps the next generation could be shocked by the Dursleys' cold and unfriendly nature. They might try to change the Dursleys' views on magic.
The children of the main Fairy Tail characters like Natsu and Lucy's kid, or Gray and Juvia's offspring could be main characters. They would inherit some of their parents' powers and personalities.
In addition to Harry and the Dursleys, there could be some Hogwarts professors involved. For example, Professor Dumbledore might play a role. He could be the one who arranges for some sort of exchange or interaction between Hogwarts and the Dursleys. Also, Hermione and Ron could be main characters if they accompany Harry to meet the Dursleys or are involved in the events that bring Hogwarts to the Dursleys' doorstep.
Obviously, Teddy is one of the main characters. He is likely to be the one driving the story forward with his actions and reactions.
The children of the original characters could be the main ones. For instance, Riley and Lucas's child could be a central character. They would inherit some of their parents' traits but also have their own unique personalities.
Often, the children of the original characters are main characters. For example, James Potter, Albus Potter, and Lily Potter. Scorpius Malfoy is also likely to be a main character as his family has a complex history with the Potters. Then there are the new friends they make at Hogwarts, like Rose Weasley. These characters inherit the legacies of their families and face new challenges in the wizarding world.
The children of Rachel and Finn could be main characters. Maybe their son or daughter inherits Rachel's amazing singing talent and Finn's charm. They could struggle with living up to their parents' legacy in the glee club.
Definitely the children of Ladybug and Chat Noir. They would inherit the Miraculous and have big shoes to fill. Their personalities could be a mix of their parents' traits, like the daughter being as brave as Ladybug and having Chat Noir's sense of humor.
There could also be a character who is not directly related to the original ninja but has a special connection to the elemental powers. Maybe a young girl who discovers she has an ability to control an element that was previously thought to be lost. She would bring a fresh perspective to the group of the next generation and have to prove herself to the others.
One possible main character could be the son or daughter of Iron Man. They might inherit his genius and some of his tech - savvy ways. They could also have a bit of his cocky attitude at the start but grow into a true hero throughout the story.