Because Danny is the origin of Dan. He has a direct connection to the whole concept of Dan in the fanfiction. His actions and choices in the main story are what lead to the existence of Dan in the alternate or future scenarios.
Danny is important to talk about in Dan fanfiction as he can be the hero who tries to fix the mess that Dan creates. His character development in relation to Dan can add depth to the fanfiction. He can show how he grows as a character in dealing with the threat of Dan, whether it's through fighting, trying to reason with him, or finding a way to change the future so that Dan never exists.
He might talk about how Dan came to be. In the fanfiction, Danny Phantom could discuss the events that led to the creation of his evil future self, Dan. Maybe he'd mention the mistakes he made that pushed him towards that dark path.
Obviously, Danny Phantom and Dan Phantom are the main characters. Danny is the original form, a heroic character, and Dan is his more evil and powerful transformation.
Obviously, Danny and Dan themselves are very popular. Danny as the main hero and Dan as his dark and complex counterpart are central to many stories. Jazz, Danny's sister, is also popular as she can provide a different perspective and often gets involved in Danny's ghost - fighting adventures in fanfiction.
Well, in some fanfictions, there are concepts like redemption arcs for Dan Phantom. It's fascinating to see how writers try to redeem such an evil character. Also, the idea of Danny using Dan's power for good in some unique situations. For example, there could be a story where Danny has to borrow Dan's strength temporarily to defeat a powerful ghost that he couldn't handle on his own.
Well, they might share some physical characteristics. Maybe in the fanfiction, Dan has the same white hair or green eyes as Danny Phantom. Also, their powers could be depicted in a similar way, like the ability to go intangible or fly.
Vlad could kidnap Danny in fanfiction for revenge. Maybe he blames Danny for something, like ruining his plans in the past. Or perhaps he wants to use Danny as a bargaining chip against the other ghosts. He might think that by having Danny under his control, he can gain more influence in the ghost world.
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.
Obviously, Danny Phantom is the main character as it's about his transformation into Dan. His friends like Sam and Tucker would also be important characters. They would likely play a role in trying to either stop the transformation or help Danny deal with the consequences. And of course, his ghostly foes might also be involved, as they could either take advantage of his situation or be affected by the power shift if he becomes Dan.
One interesting plot twist could be that Danny's friends initially don't recognize him as Dan Phantom and try to befriend him, not realizing the danger. Another could be that Dan Phantom has some hidden memories of his time as Danny that start to resurface at crucial moments, changing his evil intentions.
They could have the same basic build. Danny Phantom is usually shown as a teenage boy with a certain physique, and Dan in the fanfiction might be described in a similar way.