Perhaps some corrupted Ministry of Magic officials could be the new antagonists. They could use the power of the Ministry to target Harry and his friends. Since they have the law on their side, Harry would have to find ways to fight them without directly breaking the law. He might need to use his influence among the students and the wizarding community to expose their corruption and undermine their authority.
It could be a group of dark wizards who have a different ideology from Voldemort. They might not be interested in pure - blood supremacy but rather in power for power's sake. They could be more cunning and use more psychological warfare against Harry, which would force him to fight in new ways.
Another option is that a new magical race or entity emerges as the antagonist. For example, a race from a different dimension that has its own set of rules and magic. They might see Harry and the wizarding world as a threat or as a source of power. Harry would then have to adapt his fighting style to deal with their unfamiliar magic and tactics.
It's hard to say without more details about the fanfiction. But it could be some characters who benefit from Teddy not having rights, like power - hungry individuals or those with prejudiced views.
There could be a group of rogue witches and wizards who are against the current system in the wizarding world. They oppose the Ministry of Magic's rules and see Harry Potter as a symbol of the establishment they want to overthrow, so they become the antagonists.
Another possibility is that an overzealous member of the Order who has gone too far in their methods of fighting Voldemort. Their extreme actions might make Harry think they have become as bad as the Death Eaters, so he starts to oppose them. For example, if this member starts using unethical spells on innocent people in the name of the greater good.
In this fanfiction, perhaps Harry could use his wits more than magic. He might outsmart his enemies by setting up elaborate traps, like he did with the troll in the first year but on a much larger scale. He could use his knowledge of Hogwarts' secret passages to his advantage, leading his foes into dead - ends.
It could be a new group of rogue ninjas who have a vendetta against Konoha. They might be led by a powerful and cunning ninja who has a personal grudge against Naruto or the village in general.
It could be a group of power - hungry humans who see the Laguz as a resource to be exploited. They might want to use the Laguz's unique abilities for their own gain, like using their strength in labor or their magical abilities for warfare.
Well, obviously Harry Potter himself is a main character. He is the one who brings his British wizarding skills and experience to the New York setting. Then there might be some American wizard characters. For example, a young and brave American witch who helps Harry understand the differences in the American magical world. There could also be a powerful American wizard who acts as a mentor or guide to Harry in this new environment.
It could be a group of rogue ninja who are after the Uzumaki's powerful sealing jutsu. They see it as a way to gain unlimited power and rule the ninja world, so they target the Uzumaki clan, which makes Naruto their main enemy as he stands in their way.
One possible plot could be Harry facing a new dark wizard threat in the heart of New York. Maybe this wizard is using the city's chaos and large population to hide his evil deeds. Harry would have to navigate the unfamiliar streets, deal with muggles unknowingly in his way, and figure out how to use his magic without breaking the Statute of Secrecy on a much larger scale than in London.
The Sith, definitely. They are always the main antagonists in the Star Wars universe, and in this fanfiction, they would be the ones trying to keep Anakin on the dark side. Palpatine, in particular, would be a major antagonist as he was the one who manipulated Anakin in the original story.