Another possibility is that the wizarding establishment itself could be the antagonist. They might be afraid of Harry's new form and try to isolate or even eliminate him. After all, a hollow - like Harry could be seen as a threat to the normal order of the wizarding world, and they might not want to take any chances, so they would oppose Harry at every turn.
In this fanfiction, perhaps it starts with a strange prophecy related to Harry becoming a hollow as a baby. The wizarding world might be in chaos because they don't understand what this means for the future. As a baby - hollow, Harry could have some special powers or abilities that are unknown to normal wizards. His friends at Hogwarts later on might discover his true form gradually and help him to either reverse it or use it to fight against a new, greater evil in the wizarding world.
Some dark wizards or witches who are looking for power might see Harry as a tool or a target. If they think they can harness the power of a hollow - Harry, they would try to capture him. This could lead to many battles and challenges for Harry as he grows up in this strange new form.
In this fanfiction, perhaps Harry's hollow state gives him some unique powers that are different from his normal wizard abilities. For example, he could have enhanced speed or the ability to disappear in a different way. And Voldemort might sense this strange new power and be both intrigued and threatened by it.
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.
Maybe there could be a new antagonist, a dark shapeshifter who is jealous of Harry's power. This antagonist could be an old wizard who has been in hiding for centuries, waiting for the right time to emerge and take over the wizarding world. He sees Harry as a threat to his plans and tries to eliminate him.
Another possibility is a corrupted member of the Ministry of Magic. They might be trying to use Harry as a scapegoat to gain more power within the Ministry. They could be working with the Death Eaters or acting on their own, but they would be a significant obstacle for Harry as he tries to stay alive and free while on the run.
Perhaps a new dark wizard or witch who has infiltrated the Ministry of Magic and made sure Harry was sent to Azkaban. This antagonist could be working towards a bigger plan to take over the wizarding world and sees Harry as a major threat even when he's locked up.
It could be Voldemort. He might see Harry's new waterbending ability as a threat and try to find a way to counter it. Maybe he would seek out dark magic related to fire to oppose Harry's water powers.
It could be a new dark wizard who has risen to power. This wizard might see Harry as a threat to his plans, so he manipulates the situation to get Harry exiled.
Another possibility is a corrupted magical creature that has been influenced by dark magic from the 'sociates'. This creature could be intelligent and manipulative, causing chaos among the students at Hogwarts and the wizards in the 'sociates'. It would be difficult to defeat as it has unique abilities that are not well - known to either group.