A really cool plot could be that Sam is given a rare plant seed by a mysterious figure. When she plants it, it grows into a giant, sentient plant. Danny at first is wary of it, but they soon find out that the plant is actually friendly and has some knowledge about Danny's ghost powers. Together, they use the plant's abilities to fight off a powerful ghost that is threatening their city.
A plot could be that a magical spell gone wrong makes Sam and Danny age down to babies. Tucker discovers that there's a time limit to reverse the spell. Meanwhile, the two babies are exploring the world in a very different way, seeing things from a new perspective. This could lead to some heartwarming and funny interactions with their families and friends as they all try to solve the mystery of the spell.
In this fanfiction, it could start with Danny Phantom turning evil due to an experiment gone wrong. As for Sam, she has always had a special relationship with plants. The evil Danny might start to use plants as his minions, controlling them to do his bidding. Sam, seeing this, decides to take matters into her own hands. She begins to study ancient plant lore to find a way to break Danny's control over the plants. Along the way, she might face many challenges, like Danny sending his plant - minions to attack her. But she doesn't give up, as she is determined to save Danny from his evil state and restore the balance between the supernatural and the natural world.
One plot idea could be that they have to deal with the challenges of being young and pregnant while also facing Danny's ghost - fighting duties. Maybe they have to keep it a secret from their families and friends at first, adding a layer of mystery and stress. Another idea is that there could be some sort of ghostly influence on the pregnancy, either positive or negative, which leads to unexpected developments in the story.
One idea could be that Danny and Sam travel back to the founding of Amity Park and accidentally change something small that has huge consequences in the present. For example, they might stop the first settler from building in a certain spot, which then changes the layout of the whole town and where ghosts can enter from in modern times.
One interesting story could be about Danny and Sam going on a ghost - hunting adventure together. They might discover a new type of ghost that has unique powers and they have to figure out how to defeat it using their combined skills.
One popular Danny Phantom, Plant, and Sam fanfiction might be 'Green Ties' which explores Sam's connection to plants in the context of the Danny Phantom universe. It delves into how her love for nature and her relationship with Danny play out with plant - related elements.
One idea could be that Danny and Sam have kids in the future and their kids inherit Danny's ghost powers. The story could focus on how they learn to control their powers while also dealing with normal kid stuff like school and making friends. It would be interesting to see how Sam, as a mom, guides them through this.
One possible plotline could be Evil Danny trying to corrupt Sam. He might use his powers to manipulate situations and turn Sam against their friends. For example, he could make it seem like their friends are in danger when they're not, just to make Sam question their loyalty.
One idea could be that Danny discovers a new ghostly power that he uses to take revenge on a group of bullies who have been tormenting his friends. He could use his intangibility in a creative way, like sneaking into their lockers and making their things disappear.
One idea could be Danny Fenton having to deal with a new ghostly enemy that can manipulate time, making his double life as Danny Phantom even more chaotic. Another could be Danny trying to balance his high school life, including exams and relationships, while also facing a ghost invasion that only he as Danny Phantom can stop. Or perhaps a story where Danny Phantom discovers a hidden power within himself that has a connection to his human side, Danny Fenton.