It could be the corrupt prison guards or wardens. They might be in league with dark forces outside Azkaban. They oppose Harry and Ginny because the couple is getting too close to uncovering their illegal activities within the prison, like smuggling dark artifacts or collaborating with Death Eaters.
There could be a new dark wizard character created specifically for this fanfiction. This wizard has a vendetta against Harry and Ginny and manages to get them sent to Azkaban. Once they are in, he uses all kinds of dark magic and tricks to stop them from escaping and revealing his true identity and plans.
The Dementors could be the main antagonists. They are always a threat in Azkaban, sucking out the souls and happiness of the prisoners. They would be constantly trying to get at Harry and Daphne.
It could be Voldemort or his remaining Death Eaters. Since they have always been the main antagonists in the Harry Potter universe, in this fanfiction they might still be the ones causing the danger that leads to Harry's sacrifice for Ginny.
The main antagonists could be the Death Eaters still loyal to Voldemort. They would be trying to undermine Harry and Ginny's efforts to protect Hogwarts and the wizarding world.
Perhaps the main antagonists are some of the old - fashioned wizarding council members who believe in arranged marriages for political stability. They force Harry into the marriage with Ginny without considering their feelings, and they would oppose any attempts by Harry and Ginny to break free from the forced union.
It could be a new dark wizard that they see in the future. Maybe someone who was overlooked during the fight against Voldemort but has been secretly gathering power.
In some cases, it could be the Ministry of Magic itself. If the Ministry is under the control of corrupt officials, they might force the marriage for their own political gain. They could use Harry's popularity and Ginny's family connections to further their own agendas, and thus become the antagonists that Harry and Ginny have to fight against in the fanfiction.
In some fanfictions, an ancient evil entity that has been awakened might be the main antagonist. This entity could be after Ginny because she has some special power or connection that the entity wants to use for its own dark purposes. And of course, there could be a jealous wizard who wants to get rid of Ginny to have Harry all to himself.
In some fanfictions, there could be a new dark wizard or witch emerging. They see Harry and Ginny's unborn baby as a potential threat to their future plans of taking over the wizarding world. So, they start plotting to harm Ginny or the baby. This new antagonist could have a group of followers who help them carry out their devious plans, making Harry and Ginny's pregnancy a time of great danger and uncertainty.
The Dementors first and foremost. They don't really care about justice or innocence, and their actions are a form of torture. Then, there could be a new dark wizard or group that has emerged and wants to use Harry's situation to their advantage. They might have bribed the Azkaban staff or found a way to get in and torture Harry for their own purposes, like getting information about the Order of the Phoenix or just to create chaos in the wizarding world.
Voldemort or his remaining Death Eaters are often common antagonists. Even after his defeat in the original story, in dark fanfiction, they could come back in some form to target Harry and Ginny. For example, a Death Eater who escaped justice might seek revenge on Harry by harming Ginny.