It could be a diplomat from another country. Their kidnapping would create an international incident and there would be a lot of diplomatic pressure and intrigue involved in the story. There would be different parties trying to rescue them or use their kidnapping for political gain.
The main character could be an ordinary citizen who just happens to be in the wrong place at the wrong time and gets kidnapped. For example, a tourist visiting America.
The parents of the Beckett baby would surely be main characters. They would be in a state of panic and desperation, driving the story forward with their search for their child.
The fans themselves could be main characters. They could be shown dealing with their emotions, sharing memories of the singer, and trying to keep the singer's legacy alive. For example, a group of super - fans might form a club dedicated to the singer after the death.
A politician who drives the empire - building policies could be a main character.
There could be a young and ambitious trainer named Alex. He's on a quest to become the best Pokemon trainer in America.
A detective - style character could be the main character. This could be a human or an animal detective who is hired to find the kidnapped lemon. They might have a special connection to fruits, like they grew up on an orchard, and they feel a sense of duty to rescue the lemon. They would then go on a journey, following clues and interrogating suspects.
Suzaku could be another main character. His conflicted loyalties in the original series could be translated into this new setting. He might be on the side of the British at first, but then have a change of heart after seeing the injustices they are committing in America. His piloting skills could also come in handy, perhaps in battles against the British forces that have advanced mecha technology, similar to the Knightmare Frames in the original 'Code Geass'.
Obviously, America would be the main character. There could also be some sort of selection committee or judges as supporting characters.
The Night King could be a great antagonist. His army of the dead and his own powerful magic would pose a huge threat to Captain America. Cap's used to fighting human foes, but the Night King is something entirely different.
Hermione could be one. She might follow Harry to America to continue their adventures together. She could help Harry adjust to the new place with her intelligence and knowledge.
It could be a regular commuter as the main character. Just an ordinary person going about their day on the double decker when they get kidnapped. This makes the story relatable as it could happen to anyone.