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.
It could be that someone in their neighborhood, like Buford, teases Phineas about his inventions being silly. Usually, Phineas just shrugs it off, but this time it gets to him. He gets mad and decides to prove Buford wrong by creating an even more amazing invention. So, the fanfiction could focus on his journey to prove himself while being angry.
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.
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.
Maybe Phin gets mad because Ferb keeps taking over their projects without much consultation. Ferb is often very methodical and might start changing things in their grand plans, which makes Phin frustrated.
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.
Another possibility is that in the fanfiction, Phin gets mad when someone from their school, like Buford, teases Ferb too much. Phin is really protective of Ferb. So when Buford makes fun of Ferb's accent or his quiet nature, Phin could get really angry and decide to teach Buford a lesson.
Perhaps Phineas gets diarrhea right before a big event they were planning, like a huge backyard invention showcase. This causes chaos as they have to delay the event. Meanwhile, Candace, thinking this is a perfect opportunity to get the boys in trouble, tries to call their mom. But Ferb manages to keep her at bay while also taking care of Phineas.
One possible plot could be that Phineas gets dysentery right in the middle of one of his big summer projects. His friends and family then have to figure out how to take care of him while still trying to keep their daily adventures going. Ferb might take on more of the leadership role in their projects while also looking after his brother.
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.
It could be Perry the Platypus, in a really unexpected turn. Maybe Perry has to keep some secrets from Phineas for his spy work, and one day those secrets are revealed in a way that makes Phin feel betrayed. This would definitely make him mad.
One possible plot could be that Phin gets dysentery right in the middle of planning one of their big summer projects. This forces Ferb to take over the leadership role while also trying to nurse Phin back to health. The other kids might be confused at first about what's wrong with Phin, but then they all pitch in to help.