Maybe some rogue Aurors who have turned to the dark side. They are jealous of the Queen's power and wealth. They use dark magic to try and harm her. Harry Potter then has to face these former protectors of the wizarding world who have become the main threat to the Queen.
It could be Death Eaters who have a new plan. They might want to create chaos in the Muggle world by targeting the Queen. They see her as a symbol of Muggle power and order, so by attacking her, they hope to disrupt the Muggle - wizard relationship.
There could be a new group of dark wizards who have emerged. They believe that by getting rid of the Queen, they can establish a new world order where wizards rule over Muggles. They create various magical traps and curses for the Queen, and Harry Potter has to overcome these villains to save her.
It could be some of the remaining Death Eaters. They might target Amelia Bones because she is a powerful and important figure in the Ministry of Magic. Her knowledge and influence could be a threat to their plans of resurrecting Voldemort's ideology or causing more chaos in the wizarding world.
It could be some Death Eaters who have made a deal with a Wakandan traitor. They might want to steal Wakanda's vibranium to make powerful dark magic items. The Death Eaters' dark magic would pose a serious threat to Wakanda's peace and stability.
Voldemort would still be a likely villain. He is the main antagonist in the original series and his threat to Harry is always present. Even if Harry is trying to save himself, Voldemort's power and his desire to kill Harry would make him a formidable foe.
One possible plot could be that the Queen is in danger from dark wizards who want to take over the Muggle government. Harry, using his skills from Hogwarts, discovers the plot and uses magic to protect the Queen. He might use spells like Protego Maxima around Buckingham Palace.
The Death Eaters are a likely choice. They might target Susan Bones because of her family's connection to the wizarding world. For example, her aunt Amelia Bones was a well - known figure, and the Death Eaters could be after something related to her or the Bones family in general.
There could be new villains like an evil wizard - scientist who wants to use magic to create black holes and destroy planets for his own gain. Or perhaps an alien race that has a natural ability to drain magic from wizards and witches and they see the Hogwarts students as a source of power.
It could be a dark wizard who has a vendetta against goblins. This wizard might see goblins as a source of power or wealth and wants to enslave them. So when Harry tries to save the goblin, this dark wizard becomes the main obstacle.
It could be a dark wizard who has his eyes on Cintra's powerful magical artifacts. He wants to use them to gain more power and is willing to destroy Cintra to get them.
It could be a rogue Death Eater who has escaped from Azkaban. This Death Eater might be desperate to regain his power and believes that unicorn blood will give him the strength he needs. So he sets out to capture a unicorn in the Forbidden Forest, which is where Harry comes in to stop him.
One possible main villain could be an Egyptian wizard who has been corrupted by dark magic. He might be after some powerful Egyptian artifact that can give him ultimate power. He could see Harry as a threat to his plans because Harry has knowledge of different kinds of magic from Hogwarts.