Some former Death Eaters who escaped justice could be the main antagonists. They might be plotting to bring back Voldemort's ideology in a more covert way. Another possibility could be new dark wizards who emerge, seeing the post - war chaos as an opportunity to gain power.
There could be a group of pure - blood supremacists who didn't support Voldemort but still have their own radical ideas about the wizarding world. They might oppose Harry's efforts to create a more inclusive society. Or it could be a powerful and corrupt wizard from a foreign wizarding community who sees the weakened state of the British wizarding world after the war as a chance to take over.
Perhaps a dark force that was awakened during the war but remained hidden until now. This could be an ancient evil that was accidentally released during one of the major battles. It might target Harry because he is seen as the symbol of the new wizarding order. Also, some disgruntled wizards who lost their status or power during the war might form a group to oppose Harry and his allies.
Some of the pure - blood supremacist families who still hold grudges against Harry could be antagonists. They might not approve of Daphne getting involved with Harry, seeing him as a threat to their traditional values. For example, some of Daphne's extended family members who are still stuck in the old ways could try to break them up.
It could be some rogue ninjas who were against the peace treaties after the war. They might form a group and try to disrupt the newly - established peace in the ninja world.
It could be some remaining monsters that survived the giant war and are now seeking revenge. They might target Nico specifically because of his role in the war. Another possibility is some rogue demigods who didn't like the outcome of the war and blame Nico for it.
Another option for antagonists could be a new organization that rises in the power vacuum left after the war. They might have different ideologies from Naruino and his village. For example, they could believe in total domination rather than the peace and co - existence that Naruino stands for. They would see Naruino as a threat to their plans and try to eliminate him.
The antagonists could be a group of corrupted Ministry of Magic officials. They have turned to the dark side for power and are trying to rewrite the laws to suit their evil plans. Harry, being the hero, sings 'This is War' to expose their wrongdoings and fight against their tyranny.
There could be a new group of villains who emerged from the shadows. They might have been observing the war and learned from the mistakes of previous antagonists. They could be more powerful and cunning, perhaps using new forms of jutsu or technology that were developed during the war. Maybe they are after some ancient artifacts or secrets that were unearthed during the Fourth Great Ninja War.
The Ministry of Magic could be the antagonists. Since Harry is in a gang, they might see him as a threat to the order of the wizarding world. They could send Aurors after him and his gang mates, trying to arrest them and break up the gang.
It could be the traditional God of War villains like the gods who oppose Izuku. Maybe Ares, if he's still around in the fanfiction's version of the God of War world, would see Izuku as a threat and try to stop him.
There could be an ancient evil force that awakens because of Luffy's new powers. This force was once sealed away but now wants to destroy Luffy and everything he stands for, seeing him as a disruption to the balance it once knew.
The Thalmor could be great antagonists. Their strict control over magic and their anti - human beliefs could put them at odds with Harry. They might see his different form of magic as a threat to their power in Skyrim.