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.
Well, Naruto could visit the world of Phineas and Ferb. He might be really confused by all the crazy inventions that Phineas and Ferb create every day. And he could show them some of his ninja skills, like running up trees or making clones. Maybe Ferb would be interested in learning some ninja techniques to add to his own knowledge of building things.
The character interactions are also fascinating. Picture Naruto trying to explain ninjutsu to Phineas and Ferb. Or Isabella being amazed by Sakura's strength. These crossovers allow for a lot of humorous and heartwarming exchanges between the characters from the two very different shows. Another interesting part is how the fanfiction writers often have to create a new kind of logic to make the two worlds coexist. For example, how the technology in Phineas and Ferb's world might interact with the magic - like chakra in Naruto's world. It's a creative challenge that often leads to some really cool and innovative story ideas.
One popular naruto phineas and ferb fanfiction might be the ones where the characters from both worlds accidentally cross paths. For example, there could be a story where Naruto somehow ends up in the world of Phineas and Ferb and has to figure out how to get back while also dealing with the wacky inventions and situations there.
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.
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.
I really like the fanfiction 'Phinbella's Hidden Moments'. It focuses on those little, unspoken moments between Phineas and Isabella. It's filled with cute interactions and the writing style makes it a really enjoyable read. It gives a different perspective on their relationship, highlighting the subtleties that might not be as obvious in the original show.
For Isabella, you could develop her character by showing her growth in leadership skills. For example, in a fanfiction, she could take charge of a big project in the town and learn how to delegate tasks and handle pressure. This would make her more than just the girl with a crush on Phineas.