Maybe the main antagonist is an overzealous doctor who doesn't really care about Johnny's well - being but is more interested in using him as a guinea pig for some experimental treatments. This doctor could constantly push Johnny into procedures that are more for the doctor's own gain than for Johnny's recovery.
The antagonist could be an anonymous figure who was somehow responsible for Johnny's injury in the first place. This person might be trying to cover up their tracks and prevent Johnny from finding out the truth. They could be spreading false rumors about Johnny or causing other problems in his life to keep him distracted from the real cause of his injury.
One possible plotline could be that Johnny gets injured during an extreme sports event. His friends then rally around him, and through their support, he overcomes both his physical pain and some internal demons he was facing. Maybe there's a love interest among his friends who realizes their true feelings while taking care of him.
The sorceress Sindel could also be a main antagonist. She may believe that the relationship between Johnny and Mileena goes against the natural order of things. She could use her powers to create illusions and misunderstandings between them. Also, she might be trying to protect some ancient Outworld secret that she thinks Johnny Cage is getting too close to through his relationship with Mileena.
It could be Dr. Colosso. He's always causing trouble for the Thundermans. Maybe he devises a new plan to take down Max specifically, and in the process, Max gets hurt. Dr. Colosso might think that by hurting Max, he can weaken the entire Thunderman family.
It could be an older male character who has improper intentions towards Baby because of her innocence. He might see her virginity as a conquest and constantly try to undermine Johnny's relationship with her.
It might be an accident - causing character. This could be someone who is clumsy or irresponsible. Their actions, even if not intentional, end up causing harm to Judy. For example, they might accidentally start a fire in a place where Judy is, and she gets burned in the process.
It could be Hawk Moth. Since he's always after the Miraculouses, he might sense the presence of Tikki and Plagg with the hurt reader and try to manipulate the situation to get the Miraculouses. He could use the reader's vulnerability against them, trying to convince the reader that giving him the Miraculouses would solve all their problems.
Maybe a jealous fellow singer could be the antagonist. They might be envious of Ariana's success and popularity. So, they could spread mean comments about her on social media, which really gets to Ariana and hurts her emotionally. They could also try to sabotage her performances or career opportunities in the fanfiction.
An alien who is against the idea of humans and aliens working together might target Alex. He could be seen as a key figure in the human - alien alliance in Roswell. So, the alien antagonist tries to hurt him to break up the alliance and send a message that humans should stay out of alien affairs.
In some cases, it could be a corrupted lost boy. Maybe one of the lost boys has turned bad due to jealousy or a desire for power. He sees the hurt girl as an easy target to assert his dominance or to gain something from her, like her friendship with Peter Pan. So he becomes the main source of trouble for the girl in the fanfiction.
His best friend. This person might be by his side throughout his illness, bringing him comfort, and trying to keep his spirits up. They could also be the one who helps Johnny find the best doctors or alternative treatments.