Miu could be a key character. She might be the one who stays by Kenichi's side constantly, using her skills to keep him as comfortable as possible while he's sick.
Shigure could also play an important role. Given her knowledge and experience, she might be the one trying to diagnose Kenichi's illness and come up with a treatment plan.
Maybe Kenichi gets sick just as a big tournament or important event is about to happen. His team has to decide whether to participate without him or wait for his recovery. This creates a lot of tension and internal conflict within the group.
One reason could be to understand the fans better. By seeing what fans create in fanfiction, the characters can get a sense of what the audience loves about them, what they want more of, or what aspects of the story the fans are most interested in. This could help the characters, in a meta - sense, to better fit the expectations of the fans and perhaps influence the direction of future official stories. For instance, if a lot of fanfictions focus on a particular relationship for a character, it might make the character more aware of the importance of that relationship in the eyes of the fans.
In this 'jace gets sick fanfiction', Jace is the one who gets sick, of course. But there might also be his family members. For example, his mother could be a very important character, hovering over him, making sure he takes his medicine and gets enough rest. And perhaps there is a wise old doctor in the story, who tries different remedies to cure Jace.
Obviously Elsa herself would be one of the main characters since she is the one getting sick. Anna would also be a main character as she would be very concerned about Elsa's condition and would do everything she can to help. Olaf could also play an important role, using his cheerfulness to try to make Elsa feel better.
Obviously, Bendy would be one of the main characters since he's the one getting sick. His friends like Boris and Alice would also be main characters as they would likely be involved in taking care of him or finding a cure.
Mario would most likely be a main character too. He's Luigi's brother, so he'd be there to take care of him or at least be really worried about him. Princess Peach could also be a main character. She might visit Luigi when he's sick and bring some kind of magical item that could potentially help him get better.
The masters of Ryozanpaku are very interesting to write about. For example, Akisame Koetsuji. You could write a fanfiction where he takes on a special mission outside of the dojo, using his unique martial arts skills. Or a story where he trains a new student with a completely different personality from Kenichi, showing his adaptability as a teacher.
His best friend could be one of the main characters. They would likely be very concerned and play a big role in his recovery.
Harry and Ron would most likely be main characters. They are Hermione's closest friends, so they would be involved in her life change. They might be the ones to give her advice or react to her new appearance.
It could be a powerful gamer from another dimension who wants to take over the Kenichi world. He sees it as just another level in his never - ending game of conquest and has no regard for the lives and values of the people there.