She could travel to different alternate timelines. For example, a timeline where Danny never became a half - ghost. Dani's presence in such a timeline could cause all sorts of chaos as she tries to figure out how to get back to her own timeline while also dealing with the differences in this new world.
Dani might time travel to an era in the Danny Phantom universe's past where ghosts were more powerful or had different relationships with humans. This could lead to her having to form alliances with ancient ghosts to solve a problem that is affecting her own time.
One possible story could be that Dani travels back in time to prevent a major disaster in the Danny Phantom world. Maybe she goes back to the time when the portal was first opened and tries to stop it, which would change the entire course of events in the series.
One idea could be that Danny and Ember accidentally travel back to the founding of Amity Park. They could witness the early settlers dealing with strange ghostly phenomena and have to figure out how to not disrupt the timeline while also helping the settlers. Another plot could be that they travel to a future Amity Park that is overrun by a new type of ghostly threat, and they bring back knowledge to prevent it in their own time. Or they could travel to a time when Danny's parents were kids and see how their obsession with ghosts started.
One interesting Danny and Dani fanfiction could be about them teaming up to fight a new and powerful ghost. They use their combined powers and unique abilities to outwit the enemy. For example, Dani might use her speed while Danny uses his more powerful ghost ray.
One possible plot could be that there's a school talent show. Danny and Dani both decide to enter, but they want to keep their singing abilities a secret from the other because they're not sure how the other will react. As they practice in secret, they start to discover new things about themselves and their powers through the music.
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.
One idea could be that Danny gets an illness that affects his ghost powers. For example, he might start losing control over his intangibility or invisibility. This could lead to some really comical or dangerous situations. Another idea is that his friends try to find a cure for his strange ghost - related illness while also keeping it a secret from the town.
Perhaps in the fanfiction, Danny discovers that there is a threat in the past that could change the future in a very bad way. He, along with Sam and Dani, time - travels to stop it. Along the way, they meet younger versions of their friends and family. Dani starts to understand more about her family's history and her own place in it. Sam has to deal with the differences in the past's social norms compared to their time. And Danny battles villains from the past who are trying to take over the future using the knowledge of time travel they accidentally introduced.
Well, here's a plot. In a strange turn of events, Danny's ghost powers start to interact with alicorn magic in unexpected ways. This causes chaos in both worlds. The alicorn and Danny must work together to figure out how to stop the strange energy fluctuations. They might have to travel to different dimensions within their universes to find a solution. Along the way, they meet other characters from both franchises who either help or hinder their progress.
One idea could be that Danny is experimented on by a new group of ghost hunters who want to find a way to permanently strip him of his powers. They use some high - tech equipment that has unforeseen effects on his ghostly half, causing him to have strange new abilities. Another could be an experiment gone wrong in the Ghost Zone that Danny has to deal with the consequences of, like accidentally creating a new type of ghost. Maybe Danny himself decides to conduct an experiment to see if he can control his transformation better, but it backfires and he gets stuck in his ghost form for a while.