Sodapop might be there too. He could be the one who tries to comfort Johnny after he gets beat up. Sodapop has a kind heart, and he would likely be very sympathetic towards Johnny's situation and try to make things better between the two warring parties.
Well, perhaps Johnny made some mistake that angered Jally's group. Maybe he accidentally insulted Jally or got in the way of something Jally cared about. So Jally, with his friends, decided to teach Johnny a lesson by beating him up.
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.
His sisters, Susan and Mary, would likely be main characters. They are smart and would surely try to help Johnny. Also, Dukey, Johnny's dog, would be important as he is always with Johnny through thick and thin.
Maybe it could be about Johnny's struggle with the disease, how he copes with the symptoms, and his journey to find a cure.
One of the main characters could be Wayne, the werewolf. He and his family could bring some fun and chaos to the story. Maybe they come to visit Johnny and Mavis, and the werewolf pups accidentally cause some trouble in the baby - proofing of the house. Also, Griffin, the invisible man, could be there. He could use his invisibility to play pranks on the couple during their pregnancy, but also be a good friend when they need help.
In the fanfiction, perhaps Johnny gets a disease that affects his superpowers. He used to be able to do all these amazing things, but now he can't. His dog Dukey might be by his side the whole time, trying to cheer him up. The disease could be something that was created by one of his enemies, and now he has to race against time to find a cure before it's too late.
Batman could be another interesting crossover. Batman's dark and brooding nature would contrast with Cage's more flamboyant style. They could be forced to team up to solve a mystery that involves some sort of magic or otherworldly elements, which would be new territory for Batman and something Cage is more used to in the Mortal Kombat realm.
It could be a new character introduced specifically for this fanfiction. Maybe a bully who has just transferred to Percy's school and takes an instant dislike to him. This bully might be jealous of Percy's popularity or has a grudge against him for no real reason other than wanting to assert dominance.
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.
Sasuke could be one. Given their complex relationship, there could be a situation where Sasuke's newfound power or different ideology leads him to fight and beat Naruto in a fan - made story.