Bellatrix Lestrange could also be a great antagonist. She could target their alliance as she is loyal to Voldemort. She might try to capture them to use as bait for other Order of the Phoenix members or to simply gain information about their plans to resist the Dark Lord.
Umbridge could be a major antagonist. If Harry and Neville are working on something that goes against her rules at Hogwarts, she would stop at nothing to punish them. Her cruel and controlling nature would make her a formidable foe to their alliance.
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.
One idea could be that Harry and Neville form an alliance to secretly train students in advanced defensive magic outside of Umbridge's notice during her reign at Hogwarts. They could use the Room of Requirement as their base.
There could be an ancient force that is awakened by the meeting of Harry and Hela. This force wants to destroy both of them because their combined powers could potentially stop it from achieving its goal of spreading chaos and destruction throughout all the realms. It would use all kinds of devious means to try and separate them and defeat them one by one.
A new dark wizard or witch could be created specifically for this fanfiction. This character could be interested in using the Huntsman's skills for their own evil purposes. If the Huntsman refuses, they become enemies. This antagonist could have powers related to dark forest magic, something that the Huntsman is familiar with but in a more malevolent form.
The Ministry of Magic could be the main antagonist. They might view Harry's fascination with Dementors as a sign of him turning to the dark side. So they try to stop him from his research and exploration of Dementors, fearing he'll use their powers for evil. After all, Dementors are associated with dark magic in the eyes of the Ministry.