The male oc could be a mysterious musician. He has a unique style of music that Phineas and Ferb are intrigued by. They collaborate to put on a huge music concert in the neighborhood, but they have to overcome obstacles like getting the right instruments and finding a suitable venue. This would involve a lot of musical creativity and problem - solving.
One idea could be Phineas and Ferb building a time - traveling roller coaster that takes them to different eras in their town's history. They could meet their ancestors and see how things were built and developed. Another plot could be that Doofenshmirtz creates an invention that accidentally swaps the personalities of all the characters. So Phineas becomes super - serious and Ferb becomes overly enthusiastic. It would be really funny to see how they deal with their new personalities while still trying to complete their daily big builds.
One idea could be to explore what happens when Phineas and Ferb build a time - traveling device. They could go back in time and meet their parents when they were kids. Another idea is a story where Perry the Platypus has to team up with Doofenshmirtz's daughter Vanessa to save the day. Maybe they discover a new villain who is more powerful than Doofenshmirtz and they have to put aside their differences to stop this new threat.
Maybe the little sister gets lost in the backyard which has been magically transformed by one of their inventions into a huge, mysterious maze. Phineas and Ferb then have to use their wits and knowledge of the backyard to find her. It could show their protective side towards their little sister.
One idea could be that Phineas and Ferb build a super - advanced lightsaber - making machine. They use their usual creativity and some 'Doo - hickey' they invented to create lightsabers with unique features like changing colors on command or having the ability to transform into other tools. Ferb could be the one who focuses on the technical aspects while Phineas comes up with the wild designs.
One idea could be a crossover with 'Kim Possible'. Phineas and Ferb could use their inventive skills to help Kim in one of her missions against Dr. Drakken. Maybe they create some high - tech gadgets for her. Another idea is a crossover with 'The Simpsons'. The boys could visit Springfield and get involved in some wacky adventures with Homer and Bart. They might even build an amazing contraption in the Simpsons' backyard.
One idea could be that Phineas and Ferb build a lemon - themed amusement park in their backyard. It could have rides made out of giant lemons, and lemonade stands everywhere. Another could be that they use lemons to power a new invention that can travel through time. And a third idea might be that they discover a lemon - based alien species and have to communicate with them using lemon - scented signals.
Perhaps Candace becomes a vampire accidentally while trying to stop one of Phineas and Ferb's experiments that goes awry. As a vampire, she has to struggle with her new cravings while still trying to be a normal teen. She might start seeing Phineas and Ferb's projects as a way to distract herself from the urge to drink blood. And the boys might have to figure out how to turn her back before their parents find out.
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.
Unique character design. If the original character has a distinct personality, appearance, or backstory, it can make the fanfic interesting. For example, an OC with a super - power that is related to the wacky inventions in the show.
A cool Phineas and Ferb fanfiction idea is where they build a machine that can swap personalities with the other characters. So Phineas becomes Perry the Platypus for a day and has to do his secret agent duties, while Perry has to come up with wild inventions with Ferb. It's a fun way to see the characters in different roles and how they would handle the new situations. There are also fanfictions that expand on the relationship between Doofenshmirtz and his daughter Vanessa, and how the events in Phineas and Ferb's world affect them.