One idea could be that Danny, as a half - ghost, is revealed to be related to a long - lost royal line of ghosts. Maybe he discovers a hidden kingdom in the Ghost Zone where he is hailed as a long - awaited prince. This could lead to all sorts of adventures as he tries to balance his new royal duties with his life in the human world.
A third idea might be that Danny has to protect a young ghostly princess who has fled from her own kingdom in the Ghost Zone. She is being chased by a powerful and evil force, and Danny's heroic nature leads him to take on the role of her protector. As he does so, he gets involved in the political intrigue of her kingdom and has to figure out how to deal with the various power - hungry characters within it.
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.
One idea could be exploring Danny's relationships in a different way. For example, how his friendship with Sam and Tucker might develop if they faced a new supernatural threat. Another could be a story where Danny discovers a new power and has to learn to control it while dealing with school and his ghost - fighting duties. And perhaps a fanfiction where Danny travels to an alternate dimension within the Danny Phantom universe.
A great fanfiction idea could be a time - travel story within the Danny Phantom Phantom Planet universe. Danny somehow gets sent back in time to before he became a half - ghost. He has to figure out how to get back to his own time while also dealing with the knowledge he has of the future. This could lead to some really interesting interactions with his past self, his family, and friends. And he might accidentally change things in the past that could have a huge impact on the future of the Phantom Planet.
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 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 exploring Danny's new role as a hero on a global scale. Maybe he has to deal with new threats that are attracted to his increased fame. Another could be delving into his relationships with his friends and family after the big events in Phantom Planet. For example, how does his relationship with Sam and Tucker change now that he's a more public hero?
You could write a fanfiction where Danny Phantom discovers an ancient ghostly artifact that is actually a highly advanced piece of technology from an extinct ghost civilization. He has to figure out how to use it, and in the process, he learns a lot about the history of the ghost zone and how his powers might be related to this long - lost technology. This could lead to some really cool adventures as he tries to keep the artifact out of the wrong hands.
One idea could be a crossover where Danny Phantom and Ultraman team up to fight a common enemy from another dimension. They could combine their powers in unique ways, like Danny using his intangibility and Ultraman his beams.
One interesting idea could be that Danny disappears due to a new powerful enemy who has found a way to nullify his ghost powers. This forces his friends and family to search for him in the Ghost Zone, leading to many adventures and discoveries.
Another concept could be that Danny gets a Christmas wish from a magical being. But the wish has unexpected consequences that start causing chaos in Amity Park. He then has to find a way to reverse the wish while still enjoying the Christmas spirit with his family and friends.