Who could be the main antagonist in 'Harry leaves his parents fanfiction'?

Maybe it's a group within the Ministry of Magic who oppose Harry's influence on Teddy. They might see Harry as trying to groom Teddy to be his successor in some way and want to stop them from leaving together to prevent this.

Maybe Voldemort. If the fanfiction combines the Twilight and Harry Potter universes, Voldemort might see Edward as a threat because he is a vampire and could be allied with Harry. So he tries to drive a wedge between them and make Edward leave Harry.

Voldemort's remaining followers could be the antagonists. Even though Voldemort is gone, there are still some Death Eaters who are bitter about their loss. They might see Harry leaving with Fleur as an opportunity to get revenge on Harry or to use Fleur as a hostage to gain power. They could try to ambush them on their journey, creating a lot of dangerous situations for Harry and Fleur.

If Harry's parents are the ones who betrayed him, they could be considered the main antagonists in a sense. They created this false image of themselves as loving parents when in fact they had ulterior motives. Another possibility could be a character who was in on the parents' plan and now is trying to cover it up or continue the malicious work they started. Maybe it's an old acquaintance of his parents from their days at Hogwarts who is now out to stop Harry from finding out the truth.

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.

It could be a powerful underworld lord who doesn't want Naruto to leave. This lord might see Naruto as a threat to his rule in the underworld.

An ancient and powerful Pokémon with the ability to influence Pikachu's mind could be the antagonist. It might see Pikachu's potential and wants to use it for its own dark purposes. By making Pikachu leave Ash, it can start to manipulate Pikachu more easily.

One possible scenario could be that Harry leaves his parents due to a mysterious prophecy that he needs to fulfill away from them to keep them safe. Another could be that he discovers a hidden power within himself that he can't develop while staying with his parents, so he decides to leave.

It could be someone from the village who doesn't want Naruto to discover the truth about his parents. Maybe they fear that if Naruto finds out certain secrets, it will disrupt the power balance in the village.

It could be a dark wizard from the Fairy Tail world who sees Naruto as a threat because of his unique powers. This wizard might try to stop Naruto from leaving, fearing that he'll take the knowledge of Fairy Tail's magic to other places.