A group of rogue Death Eaters who didn't believe in Voldemort's defeat could be the antagonists. They might target Harry as he is trying to go his own way, seeing him as a symbol of the old order they want to destroy. They could set up various traps and attacks on Harry during his solo adventures.
One idea could be Harry exploring a new magical realm on his own and learning unique spells there. Maybe he meets a group of outcast magical creatures who help him on his journey.
It could be Voldemort. If this story is set during the time when Voldemort was still a threat, he might be trying to stop Harry from getting home. Maybe he has a plan that involves keeping Harry away from the safety of his home so that he can be more easily attacked or manipulated.
It could be a new dark wizard who has a vendetta against Harry. This wizard might have created a spell specifically designed to target Harry's emotions, as he knows that Harry's strength often comes from his love and friendship. By taking away his emotions, he hopes to weaken Harry and make him easier to defeat.
Voldemort could be the main antagonist. He might have seen the potential in Harry's twin and decided to get rid of the twin to weaken Harry or use the twin for his own dark purposes.
Voldemort could still be the main antagonist. Even if Harry leaves his parents early, Voldemort might be hunting for him due to the prophecy. His dark presence would be a constant threat, driving Harry's actions and the plot of the fanfiction.
It could be a jealous wizard who feels that Harry and his sister are getting too much attention in the wizarding world. This wizard might try to undermine their achievements and even put them in danger.
It could be a dark wizard who is jealous of the Potter family's fame and power. He sees Lemon as an easy target to get to Harry and plans to use her to lure Harry into a trap.
Another possibility is a group of Death Eaters who have broken away from Voldemort's control and formed their own faction. They see Harry as a symbol of the old regime that they want to overthrow. They target Harry, but his parents, with their knowledge of the old ways and their love for Harry, are able to save him from this group's attacks.
A new dark wizard who has emerged from the shadows, someone who idolized Voldemort and is trying to pick up where he left off. This antagonist could be using new and more insidious magic that Harry has to figure out how to combat as an Auror.
A power - hungry noble could be the main antagonist. This person might be jealous of the Queen's position and wants to overthrow her, and sees Harry as an obstacle in his way.