A really cool plot might involve Candace teaming up with an unexpected character to bust Phineas and Ferb. For example, she could team up with Doofenshmirtz. They both have their own reasons for wanting to stop Phineas and Ferb's projects. But as they work together, they start to see things from a different perspective and might even end up becoming friends with Phineas and Ferb instead.
Candace is obviously a very popular character as the whole concept often revolves around her. She's the one with the mission of busting Phineas and Ferb. Phineas and Ferb themselves are also really popular. Their crazy inventions and carefree attitudes are what drive Candace's need to bust them in the first place.
One interesting plotline could be how Phineas and Ferb deal with the aftermath of getting busted. Maybe they have to find a way to convince their parents that their wild inventions were actually for a good cause. Another could be that their friends rally around them to help prove their innocence.
Candace could be a main character. Since Ferb is sick, she might be in a bit of a dilemma. On one hand, she still has her urge to bust Phineas if he does something out of the ordinary while taking care of Ferb. But on the other hand, she also cares about her brother and might be more lenient or even helpful.
The parents, Linda and Lawrence, are also important characters in this type of fanfiction. Their reactions to finding out about Phineas and Ferb's activities can vary greatly. Some fanfictions might show them being really angry at first but then realizing that their sons' creativity should be encouraged rather than punished. And of course, their friends like Isabella, Baljeet, and Buford can play important roles too, either by helping the boys or having their own unique reactions to the situation.
Well, it could be about how Phineas tries his best to take care of Ferb. Maybe he builds some crazy contraption to make Ferb feel better, like a super - advanced bed that can do all sorts of things to ease Ferb's illness.
Well, perhaps in the fanfiction, Phineas gets mad because Ferb accidentally ruins one of their big inventions. Maybe they were building a huge roller coaster and Ferb made a miscalculation that made the whole thing collapse. Phineas, who is usually calm and positive, gets really frustrated and yells at Ferb for the first time.
Definitely Doofenshmirtz. He could mess with one of Phineas and Ferb's inventions in a way that really ticks Phineas off. Like he might use one of his -inator devices to turn their amazing flying machine into a pile of junk. Phineas would be furious and would then set out to fix it and get revenge on Doofenshmirtz.
Doofenshmirtz could be the antagonist. He might create a contraption that directly or indirectly causes Phineas to get mad. For example, his -inator could accidentally zap away all of Phineas's blueprints for his latest project. Phineas would be understandably angry and would then see Doofenshmirtz as the source of his frustration.
One possible plot could be that someone ruins Phineas's big invention project. Maybe Doofenshmirtz creates a device that accidentally targets Phineas's backyard and destroys all the materials for his amazing new roller coaster. This makes Phineas really mad and he decides to go on a mission to get back at Doofenshmirtz. Ferb tries to calm him down but Phineas is too angry at first. Then they end up working together to not only fix their project but also teach Doofenshmirtz a lesson about being more careful with his inventions.