It could be a new dark wizard who targets Harry because of his fame. This wizard might be jealous of Harry's status as the hero who defeated Voldemort and wants to prove himself as more powerful.
The fanfic might explore how Harry's upbringing in the wizarding world affects his mental state as he ages. He could struggle with the pressure of being 'the boy who lived' and having so many expectations placed on him. For example, he might have anxiety about always having to be the hero. Also, his relationships with friends like Ron and Hermione could change as he matures and they all face different challenges in life. This could all be part of the psychological growth aspect of the fanfic.
The main antagonist could be a powerful necromancer who combines the dark arts of Skyrim with some elements of Voldemort - like magic. He could be raising an army of the undead and causing chaos across Skyrim, which Harry has to stop.
It could be a new dark wizard who has a vendetta against Harry. This wizard might use a spell to stop Harry from eating, thinking it will weaken him before a final showdown. He could be someone who was once a follower of Voldemort and now wants to take his place and prove himself by getting rid of Harry.
Maybe an ancient evil spirit that was once bound but is now trying to break free by using the summoner as a conduit. This spirit could be manipulating events from the shadows, causing chaos in the wizarding world to draw the summoner out.
A group of bandits might be the main antagonists. They see Percy as someone they can easily rob or enslave since he is all alone. Percy would then have to use his skills to outwit them.
It could be a new character. Maybe a dark sorcerer who has come to Neverland while Peter was growing up. This sorcerer wants to take over Neverland and sees Peter, with his new adult understanding and power, as the only obstacle. The sorcerer could start to corrupt the island, turning the mermaids and Lost Boys against Peter, making him fight not only for his place in Neverland but also for the very soul of the island.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
In an alternate dimension, the main antagonist could be a dark entity that has emerged from the very fabric of the dementions themselves. This entity could be using the Dementors as its minions, controlling them to do its bidding. It would be a powerful and mysterious foe, and Harry would have to discover its true nature and weaknesses in order to defeat it.
There is also a possibility that the main antagonist is the Goblet itself if it has been corrupted by some ancient dark magic. It could be summoning Harry not for a normal Tournament but to sacrifice him or use his power for some evil purpose, making the Goblet the force that Harry has to fight against.
Maybe the demon's soul itself could be the main antagonist in a way. It has its own will and desires, which are mainly to spread chaos and destruction. It could possess other characters in the story and make them act against Harry. For instance, it might possess a Hogwarts professor who then tries to undermine Harry's efforts to stop it. The demon's soul's ability to manipulate and corrupt would make it a very dangerous and menacing antagonist.