There could also be some unknown characters in the fanfiction. Maybe a secret society within Hogwarts that has been using the cupboard for their own purposes. When Hermione finds out about it, she gets caught up in their affairs. These unknown characters could be students from different houses or even former Hogwarts students who have come back for some hidden agenda related to the cupboard.
Well, Ron is a given. He's Hermione's significant other, so he'd be deeply involved in the whole process. Luna Lovegood might also make an appearance. She has an unique perspective and could bring some calmness and different kind of support. And don't forget about Hagrid. He has a big heart and might come to offer his congratulations and well - wishes in his own, endearing way.
Other characters like Ginny Weasley could be part of 'hermione fails' fanfiction. Ginny might be Hermione's confidante when she fails in a relationship, offering advice based on her own experiences. Neville Longbottom could also be involved. He could be in a situation where Hermione fails to recognize his potential or she fails to help him when he needs it most. And of course, the Weasley twins, Fred and George, could be involved. They might use Hermione's failure as inspiration for one of their pranks or they could be the ones to cheer her up in a humorous way after she fails something important.
In this fanfiction, Hermione might be shocked when she finds out about the cupboard. She could start an investigation to figure out what's been going on in there. Maybe it's a hidden place full of secrets related to Hogwarts' history.
Ron is often involved. He's Hermione's husband in many fanfictions, so he'd be there to support her during the birth. Harry might also be present, as he's their close friend. He could be the one to ensure their safety during this vulnerable time. Ginny could be another character, perhaps helping Hermione through the labor process, offering her the kind of female support that's important in such a situation.
Harry and Ron are likely to be involved. They are her closest friends, and they would surely be there for her during such an important event. They might offer moral support or even help her solve some of the problems related to the test.
Harry Potter could be involved. He might be caught in the middle of Hermione's revenge plan on Ron. He could try to be the peacemaker, but his attempts might backfire, leading to more chaos between the two. For example, he might accidentally reveal Hermione's plans to Ron, making Hermione even more determined to succeed with her revenge.
Harry Potter often shows up. Since he's their best friend, he might be caught in the middle of their divorce, trying to mediate or just being there for both of them. His presence can add a lot of emotional depth to the story as he struggles with loyalty to both Hermione and Ron.
Another character could be a new teacher or a visiting wizard. This person might recognize Hermione's potential even more than the others and offer to mentor her. They could introduce her to new areas of magic and help her develop her skills further.
Another possible main character could be Draco Malfoy. Given his Slytherin background and the connection between Slytherin and Parselmouths, there could be some interesting interactions between him and Hermione. Maybe they start to understand each other better through their shared connection to Parseltongue.
Well, Harry and Ron would most likely be main characters. They are Hermione's closest friends in the Harry Potter universe, and in this fanfiction, they would probably play important roles in supporting Hermione through her maledictus situation. Maybe Harry would use his courage and Gryffindor spirit to stand by her side no matter what, and Ron would provide some comic relief while also being a loyal friend.