Maybe there's a demon lord who has some sort of claim over the succubus. This demon lord sees Harry's binding of the succubus as an insult and a threat, and thus becomes the main antagonist. He would then send his minions or use dark magic to try and break the bond between Harry and the succubus.
Well, it could be that Harry, in this fanfic, discovers a way to bind the succubus familiar through some ancient magic he stumbles upon. Maybe he needs its powers for a particular task, like fighting a new dark force in the wizarding world.
One possible plot could be that Harry, perhaps in a search for power or knowledge, binds the succubus. But then he has to constantly battle her wiles and temptations as she tries to break free. He might use magic he's learned at Hogwarts to keep her in check.
Perhaps a group of anti - dragon fanatics within the wizarding world could be the main antagonists. They see the dragon as a dangerous and uncontrollable creature and will do anything to get rid of it, even if it means going against Harry.
It could be a powerful demon who wants to control the Succubus Queen and use her powers for evil. Since Naruto is with the Queen, he becomes a target too.
In a fanfiction, the relationship might develop in an unexpected way. Harry Potter, who is used to the more 'normal' magical creatures, meets the succubus familiar. At first, he might be repelled by her nature, but gradually he discovers that there is more to her. They could bond over shared experiences in the magical world, like facing a common enemy. However, the relationship would always be on shaky ground due to the succubus's reputation and her own powers that could be both a help and a hindrance.
Harry's character could become more cautious. Dealing with a succubus would make him realize that there are many dangerous and unpredictable forces in the magical world. He might also become more secretive as he doesn't want others to know about the succubus he has bound.
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.