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 possible main character could be a journalist. Their job would be to document the changes and report back to the American people. They could uncover secrets about this other world and expose any threats or opportunities. Another could be a Native American leader. In this new world, they could play a crucial role in reconnecting America with its roots and using ancient wisdom to navigate the new situation. And a young entrepreneur could also be a main character. They would look for business opportunities in this other world, trying to make America prosperous in a different way.
A military general could be a main character. In a world full of new threats and challenges, he would be in charge of protecting America. He might have to lead his troops against unknown enemies or negotiate with strange new powers. Also, an ordinary citizen, like a schoolteacher, could be a main character. She could show how the common people are affected by the change and how they try to survive and make a new life in this other world. She could also inspire others with her tenacity and kindness.
There could be a demonic entity in the new world. It might be awakened by Gilgamesh's arrival. This entity is used to having its way in the world and sees Gilgamesh as a new obstacle. It could possess other beings to attack Gilgamesh and use the chaos of the new world to its advantage.
Perhaps a group of elite warriors in the new world who are jealous of Naruto's unique abilities. They form an alliance to take him down because they fear he will disrupt their status quo and take over their positions of power.
Obviously Naruto himself would be a main character. He could be joined by Sasuke. In a crossover with 'One Piece', Luffy could also be a main character in the fanfiction, as he is the captain in 'One Piece' and would bring his own unique charm and abilities.
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.
They could meet MACUSA officials. These are the magical authorities in America and would likely play a role in any story set there.
It could be a powerful wizard or sorcerer in the new world who sees Ichigo as a threat to his rule. Maybe Ichigo's presence disrupts the balance of power that this antagonist has carefully maintained.
A corrupted angel could be the main antagonist. This angel, once a protector of the new world, has fallen from grace. He has amassed an army of dark creatures and is using his divine powers in an evil way. The Doom Slayer has to face this angel's holy wrath and his hordes of minions.
The Green Goblin could be a great main antagonist. Imagine in this other world, he has even more advanced gliders and pumpkin bombs. He could be ruling over a part of the city with his goblin minions, and Spider Man has to stop him from spreading his reign of terror further. His maniacal laugh and unpredictable nature would make him a formidable foe in this new world.
In a magical world, a powerful sorcerer could be the antagonist. This sorcerer views Tony's attempts to blend technology and magic as an abomination and will do everything in his power to stop Tony from spreading his ideas and inventions. He might even try to steal Tony's technology for his own dark magic purposes.
It could be an alternate version of Vlad Masters. In this other world, he might have achieved total domination over both the human and ghost realms. Danny would then have to face this all - powerful Vlad and find his weaknesses in this new context.