Well, a plot could involve Ferb and Vanessa dealing with family reactions to their marriage. Phineas and Ferb's family might be a bit shocked at first since Vanessa is older. They would then have to prove that their love is real and strong. Maybe they go on an adventure together to find a special item for their wedding, which helps the family see how well they work as a team. Also, Vanessa could introduce Ferb to a whole new world of music gigs and Ferb shows her his secret labs.
A plot in the fanfiction could be that they get married in secret first. They keep it a secret because they know people will be judgmental due to their age difference and different personalities. They have to live a double life for a while, with Ferb still working on his inventions with Phineas during the day and then sneaking off to be with Vanessa at night. Eventually, they decide to come clean and show everyone that their relationship is based on true love and mutual respect.
Doofenshmirtz could also play a role. He could accidentally create some sort of invention that either helps or hinders the pregnancy situation. For example, an invention that detects the baby's mood, but it goes haywire and causes some chaos. Isabella might be another main character. She could offer Vanessa advice on motherhood based on her own nurturing nature.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and you're likely to find some good Ferb and Vanessa married fanfictions there. Another option is FanFiction.net. There are also some Tumblr blogs dedicated to Phineas and Ferb fanfictions where you might stumble upon some great stories about Ferb and Vanessa getting married.
One possible plot could be that they face unexpected challenges together. Maybe they have to deal with the reactions of their friends and family. Ferb might be super protective and start preparing for the baby in his own unique, methodical way.
There are fan fictions that explore their relationship in a more in - depth way. For example, a story could be set during a music festival where Ferb's musical talent catches Vanessa's attention, and they start to bond over their shared love for music. It delves into how they communicate and understand each other on a deeper level.
Another plot could involve a love triangle between Van Helsing, Vanessa, and another character. This would add an element of drama and tension to the story. Maybe Vanessa has feelings for Van Helsing, but he is hesitant because of his duty as a monster hunter. And then this new character comes in and complicates things further. They would have to navigate their relationships while also dealing with the ever - present danger of the supernatural world around them.
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.
One interesting plot could be them navigating their new married life while still being involved in the shinobi world. They might have to balance their duties and their relationship, like when Naruto has to go on long missions and Sasuke has to deal with his own investigations. They'll learn to communicate better in this new situation.
One interesting plot could be them exploring the wizarding world together as a married couple, solving mysteries that were overlooked in the original books. For example, they might discover a hidden chamber in Hogwarts that has secrets related to ancient magic.
Another plot could be them starting a family. Dan might be worried about being a good father like his own father was not always the best example, and Serena would be there to support him. They could have difficulties with fertility or adjusting to life with a new baby, but their love for each other helps them get through it.
One interesting plot could be them building a life together after marriage. They might face challenges like how to combine their different combat styles into a training regime for the next generation. For example, Jaune's more unorthodox approach and Pyrrha's highly skilled and precise style could be passed on in new and unique ways.