Obviously, Slenderman could be the main antagonist. His spooky nature and his ability to cause fear and disappearances would make him a perfect fit for the role. He could be trying to spread his influence over the wizarding world, which would put him in direct conflict with Harry and his friends.
Another possibility is that there is an ancient dark magic artifact that has a connection to Slenderman. A powerful dark wizard or witch could be the main antagonist, seeking to use this artifact to bring Slenderman fully into the wizarding world. They might believe that by doing so, they can gain ultimate power. Harry and his friends would then have to stop this wizard from achieving their goal and also deal with the threat of Slenderman.
The mummy itself could be the main antagonist. It could be a mummy that was once an evil sorcerer in ancient Egypt, and now that it has been awakened, it wants to spread its dark magic and rule over both the Muggle and wizarding worlds.
It could be Voldemort still. Imagine Ares coming into the Harry Potter world, and Voldemort seeing this as a threat to his own power. He would try to either eliminate Ares or use him for his own dark purposes. Ares, on the other hand, may not take kindly to Voldemort's evil ways and so they become enemies, with Harry in the middle trying to defeat Voldemort while also keeping Ares in check.
Maybe a new character that is created specifically for this fanfiction. This character could be jealous of the attention fem Bowser is getting from both the wizarding world and Harry Potter's group. They could start causing trouble and trying to turn everyone against fem Bowser. This new antagonist could also have some connection to Bowser's original world, perhaps an enemy from there who has followed her to the Harry Potter universe.
It could still be Voldemort. Since he is the main villain in the original Harry Potter series, in this fanfiction he would see the Goddess Lemon as a threat to his plans of taking over the wizarding world. He might try to eliminate her before she can fully use her powers to help Harry and the Order of the Phoenix.
Another option could be a corrupted version of Merlin. What if he was turned evil by some dark magic? He would know all the secrets of both the Arthurian and wizarding worlds. He could use this knowledge to create chaos and try to rule over both. Harry and the good knights of Arthur would have to find a way to reverse his corruption.
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.
Perhaps a long - lost relative of the Potter family who feels they were unjustly excluded from inheriting the manor. This relative could be jealous of Harry's status and the wealth associated with the manor. They might use dark magic to try and claim what they think is rightfully theirs, leading to a battle of magic and loyalty within the Potter family and among Harry's friends.
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.
Voldemort could still be the main antagonist. Even if Demetri has some sort of connection or change in the fanfiction, Voldemort's overarching evil plans would still be a major threat to both Harry and Demetri, especially if Demetri tries to deviate from the Death Eaters' path.
The minotaur itself could be the main antagonist. It's a powerful and dangerous creature, and if it's been set loose in Hogwarts or is guarding something that Harry needs, it would be a major obstacle for him to overcome.