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.
Well, perhaps in the fanfiction, Ginny might be forced by some external power to make Harry do things he doesn't want to. For example, she could be made to persuade Harry to take part in a dangerous mission that he initially refuses. Or it could be about Ginny's overzealous attempts to get closer to Harry, which Harry feels are forced, creating a rift between them that they need to overcome.
Another development could be Ginny's perspective. She might be conflicted as well. She loves Harry, but not in this forced way. So she could be torn between her feelings for Harry and her own sense of independence. She may start to rebel against the idea too, which could lead to some interesting interactions between her and Harry as they both deal with the situation.
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.
Maybe it's a political move within the wizarding world. The Ministry of Magic might force this marriage to unite two powerful magical forces. Harry would then have to deal with his feelings for his friends like Hermione and Ron, who would be shocked. Snape, on the other hand, would have to adjust his view of Harry from a student to a partner. They could start to find common ground in their love for Hogwarts and magic in general.
One possible plotline could be Ginny leaving Harry due to the pressure of his fame. She might feel overshadowed and decides to pursue her own life away from the spotlight. Another could be a misunderstanding. Maybe she misinterprets Harry's actions or words and in a fit of anger, leaves him. Or perhaps an external threat forces her to leave to protect Harry, like a dark wizard threatening her to stay away from him so as not to be used against Harry.
One possible plot could be that there is a powerful dark wizard who forces Harry into marriage as part of a grand scheme to gain control over the wizarding world. Maybe this wizard has some ancient magic that compels Harry and he has to find a way to break free while also dealing with the political and social ramifications of this forced union within the wizarding community.
In many of these fanfictions, a common plotline is the exploration of the emotional turmoil that both Harry and Ginny go through. Harry might be conflicted between his sense of duty and his own feelings for Ginny. Ginny, on the other hand, may have to deal with the loss of agency in her relationship. There could also be a plotline where the wizarding laws and traditions play a significant role. For example, they might have to navigate through some ancient laws that enforce the marriage. And sometimes, there's a plotline where the forced marriage is part of a larger conspiracy in the wizarding world, which Harry and Ginny have to uncover and stop while also dealing with their relationship problems.
One possible plotline could be that Harry is magically transformed into a girl against his will. He then has to deal with the social and emotional changes that come with it, like how his friends react. Maybe Hermione tries to help him adjust while Ron is a bit more confused. Another could be that some dark magic forces Harry to take on a female form in order to complete a dangerous task, and he has to navigate the wizarding world as a girl to find a way to break the spell.
One plotline could be that Voldemort wants to use the marriage to gain control over Harry's special powers. Another could be that it's part of a prophecy that neither of them can avoid. Or maybe it's a political move in the wizarding world to end the war between the two sides.
One possible plotline could be Harry trying to find a way to break the forced marriage bond while secretly gathering allies to fight Voldemort.