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.
There could be a plotline where an unexpected character comes to their rescue after they get busted. This could be a long - lost relative who has some influence and uses it to get the charges against Phineas and Ferb dropped. Or it could be a rival inventor who sees the value in their ideas and decides to vouch for them in front of their parents and the authorities.
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.
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.
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 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.
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.
Another interesting OC could be a time - traveling character. This OC could accidentally land in the Phineas and Ferb's world while on a mission in time. Their presence could cause some chaos as they try to fix their time - traveling device, and Phineas and Ferb might end up helping them, leading to an adventure through different eras.
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.
In Phineas and Ferb Avengers fanfiction, a common crossover idea is the Avengers being called to the Phineas and Ferb universe because of some sort of energy anomaly. This anomaly could be caused by one of Doofenshmirtz's inventions gone wrong. Hawkeye might find himself in a strange situation where he has to use his archery skills in the wacky world of Phineas and Ferb. Hulk could be both a source of amazement and a bit of a problem as his large size might accidentally destroy some of the boys' projects. The crossovers often play with the differences in the two universes' rules and characters' personalities to create unique and fun stories.
The OC could be a time - traveler from the future. She comes back to the present to stop an event that will have a negative impact on the future. Phineas, being the curious and adventurous type, joins her on this mission. They use their combined creativity and Ferb's logical thinking to solve the mystery and prevent the disaster.
Well, they might use their powers to build even more amazing things in the backyard. Like they could create a floating city or a time - traveling machine that actually works. And then they'd have to deal with Doofenshmirtz trying to steal their powers for his own evil plans.