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.
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.
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.
One possible plot development could be Harry initially resenting the forced marriage but then gradually realizing Ginny's true worth. For example, through shared adventures in the wizarding world that they are forced to embark on together, he sees her courage and loyalty in a new light.
The relationship might also develop in a more magical way. There could be some sort of magical bond that was created when they were forced to marry. This bond makes them more in tune with each other's emotions. As they learn to understand and control this bond, their relationship blossoms. They start to have feelings for each other that are deeper than just friendship, and eventually, they might truly fall in love.
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.
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.
One possible plotline could be Harry initially resisting the marriage due to feeling forced into it, but as he spends more time with Ginny, he discovers new sides of her and gradually falls in love. For example, they might have to work together to face some new dark magic threat in the wizarding world, and through that shared experience, their relationship deepens.
Yes, there might be some interesting ones. However, the concept of forced marriage goes against the original spirit of the Harry Potter series. Some fanfictions might explore this idea in a creative way to show the characters' struggle and growth within such an unexpected situation.
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.
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.