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.
Well, it could be about Harry Potter having to face a new kind of dark magic related to the demon's soul. Maybe he discovers a hidden artifact that is infused with the demon's soul and has to figure out how to destroy it without getting corrupted himself.
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.
It could be a group of lesser - known demons. For example, there might be a demon named Zargoth who has the ability to manipulate time and space a little bit. He could use this power to save Harry from dangerous situations. Another could be Lythra, a female demon with the power of illusion. She can create false images to deceive Harry's enemies.
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.
It could be a rogue Death Eater who wants to take over Voldemort's position and sees Harry and Bellatrix's new - found alliance as a threat. For example, a Death Eater who was always jealous of Bellatrix's closeness to Voldemort.
It could be a new dark wizard who has somehow tapped into the power of both the wizarding world and the Narnian magic in a negative way. This wizard might want to rule both realms and is using dark magic from both to achieve his goals.