First, know the characters well. Research Danny Phantom and his personality traits, as well as any established characteristics of his potential girlfriends in the show. Then, create a unique plot. It could be something that hasn't been explored much in the original series, like a time - travel adventure involving Danny and his girlfriend.
Another fanfiction could be set in an alternate universe where Danny's girlfriend has some sort of superpower too. Maybe she can communicate with spirits in a different way than Danny. They could team up to solve mysteries in the ghost world and their relationship could grow stronger as they face challenges together.
In this fanfiction, it could be that Danny Phantom meets a Danny Phantom from a parallel fan - made story. At first, they don't trust each other. But as they learn more about each other's worlds, they realize that there are some common elements that are being threatened. It could be that a powerful ghost is trying to rewrite all the fanfic universes related to Danny Phantom. So, they must combine their strengths and knowledge to stop this from happening.
Well, in this kind of fanfiction, Danny could be controlled by an ectoplasmic parasite. The parasite latches onto Danny and takes over his actions. His eyes might glow a different color to show he's not in control. His friends would first be confused, then realize something is amiss and start investigating.
There could be a fanfic where Danny and Ember accidentally swap powers. This leads to a series of comical and also dangerous situations. Danny has to figure out how to control Ember's power of influencing people through music while Ember struggles with Danny's more combat - oriented powers. It could be a story full of self - discovery for both of them.
Another possibility is that he's facing some emotional turmoil. Danny has a lot on his plate. He has to keep his identity a secret from his friends and family, deal with school, and fight ghosts. All this stress could be causing him to lose his appetite. He might be too worried or preoccupied to even think about food.
One possible story could be that Danny is facing a new and extremely powerful ghost. He needs help from his friends Tucker and Sam. Maybe they need to find a rare artifact to defeat this ghost. They could start by researching in the local library, looking through old ghost lore books. Tucker uses his tech skills to try and track down any leads on the artifact's location, while Sam uses her knowledge of the occult.
Obviously, the main character would be Danny Phantom himself, but in two different forms or versions. One would be the original Danny Phantom from the canon source, and the other would be a version created by the fanfiction. There might also be his friends like Sam and Tucker who could play important roles in helping the two Dannys understand each other and deal with any issues that arise from their meeting.
He might meet a nurse who has some sort of psychic ability and can sense that Danny is different. This could lead to an interesting interaction where Danny has to be extra careful.
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.