One good fem Perry fanfiction could be 'The Adventures of Fem Perry'. In this, Perry might be reimagined as a female character with all her secret agent skills intact. She could have different interactions with Phineas and Ferb, adding a new layer of charm to the story.
Archive of Our Own (AO3) is also a great place to search. It has a well - organized tagging system. So if you search for 'Phineas and Ferb' and 'fem Perry', it should bring up relevant fanfictions. Plus, AO3 has a community that often recommends good reads in the comments.
He could be portrayed as a very clumsy human at first. Since he's used to his platypus body, he might have trouble adjusting to having hands and feet. For example, he could trip over his own two feet while trying to run.
One idea could be Perry having to navigate the human world while still trying to keep his secret agent identity hidden from Phineas and Ferb. He could end up getting a regular job as a human and have all sorts of comical misunderstandings at work.
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.
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.