One idea could be that they have a baby who inherits Phineas' creativity and Isabella's determination. The baby could start showing signs of building amazing things from a very young age, just like Phineas. And Isabella would be the encouraging and protective mother, teaching the baby about the importance of kindness and hard work.
Financial challenges could be one. They are still young, and having a baby means they need to provide for it. Buying things like diapers, food, and clothes can be costly. Also, they might face challenges in balancing their own hobbies and dreams with taking care of the baby. Phineas loves his big building projects, and Isabella has her own interests, but now they have to make time for the baby too.
It can be portrayed as a platonic, cozy moment. For example, they could be reading a book together in bed. Phineas could be turning the pages while Isabella is snuggled up next to him, listening intently as he reads aloud. They are just enjoying each other's company in a very innocent and friendly way.
It's popular because fans of the show love the idea of these two characters getting together. Their relationship in the original show has a lot of potential for development, and fanfiction allows fans to explore that potential.
One possible plot could be them building their dream house together. They might combine Phineas' creative building skills with Isabella's ideas for a cozy and unique home. For example, Phineas could construct a multi - level treehouse with all kinds of cool gadgets, and Isabella could decorate it with her favorite colors and patterns. And they could have adventures in their new neighborhood, making new friends and helping out the community as a married couple.
One possibility could be that Phineas and Ferb would immediately start to plan a rescue mission. They are known for their creative problem - solving skills. They might build some sort of high - tech gadget to track Isabella's location. Ferb would probably come up with the initial concept, and Phineas would add all the bells and whistles to it.
Phineas would be shocked at first. He would quickly shift into action mode. He might say something like 'We have to find Isabella right away!'. Ferb would nod in agreement and start gathering tools and materials for their rescue plan.
One interesting fanfiction could be where Isabella finally confesses her long - hidden feelings to Phineas in a very dramatic way. Maybe during a big event in the town, she just blurts it out, and Phineas is initially shocked but then realizes his own feelings for her too.
In 'Phineas and Ferb', Phineas and Isabella's love is a sweet and subtle story. Isabella clearly has a big crush on Phineas. She often tries to get his attention in cute ways, like when they are on their various adventures. Phineas, while not always fully aware of the depth of Isabella's feelings, really enjoys her company. Their relationship is part of the charm of the show, with little moments here and there that show their connection, like when they build things together or go on outings.
There might be a fanfiction where they go on an adventure together, like exploring a mysterious forest on the outskirts of their town. During this adventure, Phineas realizes just how much Isabella means to him and that his feelings are more than just friendship. It's a story full of excitement and the budding of their relationship.