It could be Sebastian's old enemy. Maybe this enemy has a vendetta against Sebastian and when he finds out about Harry, he decides to target him to get revenge on Sebastian.
Perhaps the antagonist could be a group or organization that opposes Sebastian's beliefs or actions. They see Harry as a threat just because of his relation to Sebastian and will stop at nothing to eliminate him and undermine Sebastian's influence.
Another option is a powerful wizard or sorcerer who wants to use Harry's unique heritage for his own dark purposes. Since Harry is Sebastian's son, he might possess some special qualities that this antagonist wants to harness, like a rare magic ability.
A powerful dark wizard who has a grudge against those with fire - related magic could be the main antagonist. He might see Harry's newfound 'Son of Fire' powers as a threat to his own plans for domination.
A new dark wizard could be the main antagonist. Maybe someone who was inspired by Voldemort's ideology but has new and more devious ways of causing chaos in the wizarding world. This antagonist could target Harry Potter's son to prove himself as a more powerful and feared dark wizard.
One possibility could be that Harry discovers his true parentage and has to deal with the shock. He might struggle to understand why Sebastian was not in his life before. And then he could embark on a journey to learn more about Sebastian's world, meeting new characters along the way.
Voldemort could still be the main antagonist. Even though Harry has the blood of Darth Vader, Voldemort's goal of taking over the wizarding world remains the same. Harry would have to use his combined powers from both his wizard and Vader heritage to defeat him.
A power - hungry merman who wants to rule over all the mer - people. He sees Harry as a threat because Harry has some special powers or a claim to the throne that he doesn't want to share.
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.
Perhaps Voldemort himself, even though he doesn't have the natural ability to shapeshift in the original story. In the fanfiction, he could find a way to mimic shapeshifting, or he could use dark magic to control other shapeshifters to go against Harry. This would make for an even more dangerous and complex conflict as Harry has to face his greatest enemy with this new aspect of his powers.
It could be a villain from Harry's world who is jealous of his new connection with the Starks. Maybe a dark wizard who wants to steal Stark technology to enhance their own dark magic.
It could be a corrupt prison warden who has a personal vendetta against Harry. This warden might make Harry's life in prison extremely difficult and try to prevent him from ever getting out, even if he's innocent.