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.
Some of the more traditional pure - blood families in the wizarding world could be antagonists. They might not approve of Harry, a half - blood, being involved with Pansy, and they could try to stop the pregnancy or break up the couple for the sake of 'pure - blood' ideals.
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.
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.
One option could be time - travelers from a different future. Maybe there are some wizards from a parallel future who want to change the past to suit their own needs. They might be opposed to Harry and Ginny's kids because they are trying to protect the original timeline. These antagonists could use some really complex time - magic to try and outwit the kids.
A new dark wizard could emerge in the wizarding world. This wizard might be jealous of Harry's fame and power and think that by getting to Lily, he can gain some sort of advantage. He could try to steal Lily's magic or use her to spread fear in the wizarding community.
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.
The Capitol could still be the main antagonists. They might see Katniss's pregnancy as a threat, either because they fear the child could grow up to be a symbol of resistance like Katniss or just because they want to maintain control over her. They could try to harm her or the baby in some way.
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.