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 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.
Obviously, America would be the main character. There could also be some sort of selection committee or judges as supporting characters.
It could be Voldemort or one of his Death Eaters. They might be using Harry as a pawn to get to Parvati for some reason, like she has a special power or knowledge that they want.
The President could be a main character. He or she would have to make decisions on how to deal with this new world. Maybe a young, idealistic politician who tries to bring American values to this strange new place. Another could be a scientist who discovers the secrets of this other world and helps America adapt.
One potential victim could be a small, weak nation - character. Since America is a powerful nation in the Hetalia universe, he might target a smaller nation that he suspects of having some sort of information or ability that he wants. Another could be a character who is an enemy spy in the fanfic's plot. America would want to kidnap them to get information. Also, it could be a character who has a valuable cultural artifact that America wants to 'protect' by taking the character who has it into custody.
Often, there will be some of Captain America's old friends or comrades in these fanfics. For example, Bucky Barnes might make an appearance. He could be like an uncle figure to the son, teaching him about the old days and some combat skills. There could also be new characters created specifically for the fanfic, like a young sidekick for the son who has different powers and a different background, adding more depth and variety to the story.
One likely main character could be the personification of Canada, like a young, polite and nature - loving character. Another could be the personification of America, maybe a confident, ambitious and diverse character. And there could also be side characters like citizens from both countries who witness and are affected by the hugs between the two main characters.
The kidnapper is also a main character. Their motives, whether it's for money, revenge or something else, are important in understanding the story. There might also be a character who tries to rescue the kidnapped person, like a detective or a family member.