Definitely his sisters. They are constantly inventing things in the show, so in the fanfiction about Johnny getting a disease, they would be at the forefront of trying to find a cure. Another character could be his dad. He might be the one to comfort Johnny when he's feeling down because of the disease. And of course, Dukey would be there to provide some comical relief during the tough times.
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.
Besides Johnny, his sisters Susan and Mary would play major roles. They could use their scientific skills to research the disease and come up with possible cures. Then there's Dukey, who would be Johnny's constant companion during his illness. There might also be some of Johnny's school friends who could either be supportive or, in some cases, be a bit afraid of catching the disease from him.
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.
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.
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.
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 common theme is identity. Johnny has to figure out who he is now as a girl. Another theme is family reactions. His family has to deal with this new version of Johnny, and it often leads to comical or heartwarming moments. For example, in some fanfics, his dad is completely clueless at first.
Well, perhaps there's a scientific experiment in Johnny's world that goes wrong. It creates a portal that sucks Naruto into Johnny's universe. Johnny's sisters think Naruto is some kind of alien at first. Naruto tries to find a way back home but gets involved in Johnny's adventures. They discover that an evil scientist in Johnny's world has plans to use ninja powers for his own evil deeds. So Naruto and Johnny, along with Dukey, have to stop him. They face many challenges along the way, like getting past security systems in the evil scientist's lair using Naruto's stealth skills and Johnny's hacking gadgets.
In a Johnny Test love potion fanfiction, the love potion could be part of a science fair project. Johnny's intention was to create a potion that would make people like each other more in general. But when he tests it on himself, he falls in love with the concept of time travel (because he was thinking about it right before taking the potion). So he starts trying to build time - travel machines to be closer to what he loves, and his family has to stop him before he causes a time - related disaster.
For a Johnny Test TG Fanfic, Johnny is the key character. However, adding some of the minor characters can make it more interesting. For instance, Mr. Black and Mr. White, the two government agents. They could be investigating the strange transformation that has occurred to Johnny. The school bully could also be included. Imagine his reaction when Johnny shows up to school in a completely different form. And don't forget about the various pets in the neighborhood. They could sense that something is different about Johnny and react in funny ways, which can add a lot of charm to the fanfic.
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 fanfiction, the concept of female growth hormone in relation to Johnny Test could be used in various ways. It might be part of a science - themed story where a female character accidentally takes a growth hormone and Johnny Test has to help deal with the consequences, like her growing out of control and causing chaos in the town.