A possible plot could revolve around the secrecy and intrigue related to the marriage law. There could be a group that opposes this law and tries to disrupt their relationship. Hermione and Bill would then have to fight against these external forces while also dealing with their own growing feelings for each other. They might have to go on a journey to find loopholes in the law if they want to have a more equal or free relationship. Along the way, they would learn more about each other's strengths and weaknesses, and their love would be tested multiple times.
The plot could involve how they met under the circumstances of the marriage law. It could be that the law forces them into an arranged - like situation at first. Hermione might be initially reluctant as she had other plans or feelings for someone else. But as she gets to know Bill, she discovers his kind heart, his knowledge of the wizarding world (especially his work with magical creatures), and his sense of adventure. Bill, on the other hand, might be attracted to Hermione's intelligence and her determination to make the wizarding world a better place for Muggle - borns. As they spend time together, they gradually fall in love and find ways to make the marriage work within the framework of the law.
One possible plot could be the challenges they face due to the marriage law. Maybe there are cultural differences between Hermione, a Muggle - born witch, and Bill, from a traditional wizarding family. They might have to deal with family expectations and the reactions of their friends. For example, Hermione could struggle to adapt to some of the Weasley family traditions, while Bill tries to make her feel welcome. Also, the marriage law might have certain requirements or implications for their magic use or their status in the wizarding world that they need to navigate together.