An ancient and malevolent spirit that was once trapped in the black puddle but has now been freed could be a major antagonist. This spirit wants to take over the queen's body and use her connection to the puddle to spread chaos across all the realms. It constantly whispers dark thoughts into the queen's mind, trying to make her lose control and become a puppet for its own evil plans.
One idea could be exploring the origin of the 'queen of the black puddle'. Maybe she was once an ordinary being who was transformed by a dark magic spell. Another plot could involve her forming an unlikely alliance with a group of adventurers who initially feared her but later realized she was key to saving their world from an even greater evil. And a third idea might be her quest to find a way to purify the black puddle she rules over, as it's slowly corrupting the surrounding lands.
Another possible antagonist could be some dark wizards who are trying to use Harry's new identity for their own evil purposes. They might try to manipulate him or use his connection to the Blacks to gain access to powerful magic or influence within the wizarding world.
It could be the pure - blood supremacists. They might see the relationship between Harry, a well - known wizard, and the Veela Queen as an abomination because they believe in the purity of wizard blood and would not accept a union with a non - wizard being like the Veela.
The descendants of the Death Eaters would make great antagonists. They might still hold a grudge against Sirius Black for his actions against Voldemort and his followers. They could target his daughter to seek revenge or to prevent her from uncovering any secrets that Sirius might have known about them.
Alois could be a main antagonist. In a high school setting, he might be the jealous rival of Ciel. He could try to undermine Ciel's position as student council president or spread rumors about him. His butler, Claude, would be helping him with his devious plans.
Voldemort could still be a main antagonist. His dark magic and thirst for power would put him at odds with Harry, especially if he sees Harry's connection to Black Widow as a threat. Another could be some of the more traditionalist wizards who don't trust Harry because of his strange heritage.
The Saiyans could also be antagonists. If they discover Black Goku's true nature and his alliance with Zamasu, they would likely oppose them to protect their race and the universe.
The Death Eaters could be antagonists. Even though Regulus turned against them, they might not forgive his betrayal and try to harm him or the school where he teaches. They could send spies or launch attacks to get revenge.
There could be an ancient evil force that has been dormant for a long time. The formation of the harem between Naruto and Moria Queen awakens this force as it senses the newfound power and unity. This antagonist would be extremely powerful and would pose a great threat to not only the harem members but also the entire world they are in, forcing them to work together to defeat it.
Perhaps there could be an organization that wants to control the Yu - Gi - Oh world. They view the Queen of Games as a wild card that could disrupt their plans. So they send their own powerful players to stop her. They might use underhanded tactics like cheating or sabotaging her decks. This would create a lot of conflict and drama in the fanfiction.