It could be a group of rogue Death Eaters who have formed their own empire after Voldemort's fall. They might be more radical and have new, even darker plans for the wizarding world.
Maybe some powerful and corrupt pure - blood families who have always wanted to dominate the wizarding world could be the main villains in this fanfic. They could use their wealth and influence to build an empire and suppress those they consider inferior. Harry Potter would then have to fight against their age - old prejudices and power - hungry ways.
Well, it could be that Harry Potter discovers an evil empire within the wizarding world that has been secretly controlling things. He then uses his magic skills and the help of his friends to fight against it. Maybe there are new dark spells and powerful artifacts involved in this fight.
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.
A powerful alien sorcerer could be the main villain. This being has abilities similar to Harry's magic but on a much greater scale. It has set its sights on Earth, and Torchwood is the only thing standing in its way. Harry has to figure out how to use his unique brand of magic, combined with Torchwood's technology, to defeat this all - powerful enemy.
In a 'harry potter infinite empire fanfic', of course Harry would be a key character as he is in the original series. However, there might be some new protagonists or antagonists related to the infinite empire. For example, a mysterious figure who holds the secrets of the empire's infinite power. Hermione might play a crucial role in deciphering the complex magic and politics of the empire, and Ron could be there for his loyalty and combat skills.
The Malfoys could be the main villains. They might have turned to even darker magic after the fall of Voldemort and formed their own secret organization. Lucius Malfoy could be using his wealth and influence to gather dark wizards and witches to create chaos in the wizarding world.
The Death Eaters could also be major villains. They might team up with some anti - SHIELD groups in the muggle world. The Death Eaters' dark magic combined with the muggle - world weaponry of these groups could pose a serious threat to both the wizarding and muggle worlds that Harry and SHIELD have to stop.
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.
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.
One potential villain could be a rogue Death Eater who escaped justice. This Death Eater might see Teddy as a symbol of the new generation that he wants to destroy. Another could be a jealous wizard who is envious of Teddy's special abilities, like his Metamorphmagus powers, and tries to harm him. Harry would then have to fight these villains to protect Teddy.
Some dark wizards who were in hiding during the main battle might emerge. They could see the post - battle chaos as an opportunity to gain power.