For a crossover with 'Avatar: The Last Airbender', Aang, Katara, Sokka and Toph would be great inclusions. Aang's airbending and his fun - loving nature could pair well with Phineas and Ferb. Katara could use her waterbending to help with any water - related problems in the crossover. Sokka could contribute his strategic thinking, and Toph with her earthbending could be involved in building or demolishing things as part of the adventure. Zuko could also be added later for some extra drama and character development.
In a crossover with 'Gravity Falls', Dipper and Mabel would be great characters to include. Dipper's curiosity and Mabel's wackiness could blend well with Phineas and Ferb's adventures. They could all team up to solve a mystery that involves both the strange happenings in Gravity Falls and Phineas and Ferb's crazy inventions. Another good character could be Perry the Platypus. Since he is a secret agent, he could have some interesting interactions with other characters from different shows in a crossover.
If it's a crossover with 'Adventure Time', Finn and Jake would be awesome. Finn's heroism and Jake's shape - shifting abilities could add a lot to the story. They could join Phineas and Ferb in building something amazing or go on a quest together. Also, Marceline the Vampire Queen could be an interesting addition. Her cool and sometimes dark personality would contrast with the more light - hearted nature of Phineas and Ferb's world. And don't forget about Princess Bubblegum, who could collaborate with the boys on some scientific projects.
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.
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.
I think Naruto would be the most interesting. He has such a unique background and abilities. His ninja skills and his journey to become Hokage would contrast so much with the laid - back, creative world of Phineas and Ferb. He could bring a whole new level of action and adventure to their world.
I think Phineas and Iron Man would be an interesting pair. Phineas is full of wild ideas and Iron Man is a genius inventor. They could bounce ideas off each other and come up with some really out - of - this - world inventions together.
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.
I really like 'Phineas and Ferb Crossover with Star Wars'. It's a really cool fanfic. In it, Phineas and Ferb end up in the Star Wars universe. They use their skills to help the rebels. Ferb is like a super mechanic for the X - wings and Phineas comes up with plans to outwit the Empire. It's full of action and adventure.
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.
Obviously, Phineas would be a main character as he is the one getting dysentery. Ferb would also be very involved as his brother. He would be there to support Phineas and probably try to come up with solutions to make him feel better. Perry the Platypus might also play an important role. Even though he has his secret agent duties, he could still keep an eye on Phineas from a distance and make sure he is safe.
There could be fanfictions that explore the relationships of the characters in their adult lives. Maybe a story about how Phineas and Isabella finally get together and face the challenges of an adult relationship. Or a story about how Ferb and his friends support each other in their different life paths as adults.
Baljeet could also be paired with Candace. His intelligence could be useful in researching ways to reverse her vampire condition. He might be initially scared of her but then realizes that she's still the same Candace underneath. And Perry the Platypus could be involved too. As a secret agent, he could be on a mission to figure out what's going on with Candace and protect the town from any potential vampire - related threats that her transformation might bring.