It's possible that they decide to take their relationship to the next level. In the Danny Phantom fanfiction world, after numerous dates, they could consider revealing their relationship to their families and friends. This could bring a whole new set of storylines, like dealing with Sam's overprotective parents or Danny's ghost - hunting friends' reactions.
The relationship could develop into a more trusting one. After so many dates, they've shared a lot, and there's a stronger bond of trust. For instance, Danny may trust Sam enough to show her a new power he's discovered.
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?
A new character could be a foreign hero who comes to Amity Park to learn from Danny. They might have different powers and a different approach to ghost - fighting. Another possibility is a scientist who is intrigued by Danny's powers and wants to study him, not in a malicious way, but out of pure scientific curiosity.
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.
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.
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.
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.