One possible plot is that Fred and Hermione accidentally meet at a wizarding club. They start to have a connection there, but then realize that the marriage law in the wizarding world has some restrictions that could affect their potential relationship. They then decide to find a way around these restrictions, which could involve seeking help from other magical beings or researching ancient magic laws.
Well, the plot might begin with Hermione being dragged to a club by her friends and Fred already being there. They catch each other's eyes and start talking. The marriage law could be something that is not really on their minds at first, but as their relationship deepens, they discover that there are laws governing magical marriages that they need to understand. For instance, there might be laws about property division or magical abilities inheritance within a marriage that they need to consider. This could lead to a series of adventures as they try to figure out if they can be together within the framework of the law.