Superman would be a great choice. Danny's ghost powers combined with Superman's super strength, flight, and heat vision could make for some really epic battles. Another good one is Batman. Danny could use his invisibility and intangibility to help Batman in his detective work and in combat against Gotham's villains. Wonder Woman is also a good option. Her lasso of truth and combat skills along with Danny's powers would create a very powerful character.
The Flash would be an exciting character to fuse with Danny. Danny could use the Flash's super - speed in combination with his ghost powers, like zipping through walls while being intangible. Another character could be Green Lantern. Danny could use his powers to create unique constructs with the power ring, and his intangibility could give him an edge in combat. And from other universes, Goku from Dragon Ball Z. Danny could learn ki control from Goku and use it along with his ghost powers to become an even more powerful fighter.
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.
Sam is also quite popular. Her goth - like personality and her strong - willed nature make her an interesting character. In fanfiction, she can be portrayed in different ways, like being even more involved in Danny's ghost - fighting activities.
A good character to introduce could be a new ghost with a very unique power. For example, a ghost that can control dreams. This character could cause a lot of chaos by invading people's dreams and Danny would have to find a way to stop it. Another character could be a human with the ability to see ghosts clearly but who has been hiding this ability. They could either help or hinder Danny in his adventures.
In Danny Phantom experiment fanfiction, Danny's parents, Jack and Maddie Fenton, are obvious choices. Their knowledge of the supernatural and their inventions could play a big role in the experiment. Then there's Valerie Gray. She could be another test subject in the same experiment as Danny, which would create an interesting dynamic between them. Dash Baxter could also be included in a unique way. Maybe he accidentally stumbles upon the experiment and his reaction to seeing Danny's powers could add some humor. And let's not forget Clockwork. He could be the one who foresaw the experiment going wrong and tries to guide Danny through the chaos.
For Danny Phantom Halloween fanfiction, Clockwork would be an interesting character to include. He could play a mysterious role, perhaps knowing about some impending doom on Halloween and guiding Danny in a cryptic way. Ember McLain is also a great choice. She could use her music powers to create a spooky atmosphere all over town during Halloween. And let's not forget about Skulker. He could be on a special Halloween hunt for Danny, seeing it as the ultimate prize to catch the half - ghost on this spooky night.
In addition to the ghosts, there might be some human characters that are important in the 'phantom city danny phantom fanfiction'. They could be Danny's friends or family members who get involved in the events of the Phantom City. Their reactions to the supernatural world and their interactions with Danny and the ghosts would add an interesting dynamic to the story.
Definitely Jazz. Since she's Danny's sister, her reaction to Danny possessing someone or being possessed would be really interesting. She could be the one to figure out what's going on and try to help him.
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.
Sam and Tucker are also quite popular. They often play important roles in helping Danny deal with his TG (transformation - related) situations. Sam might use her goth knowledge or her strong will to assist Danny, while Tucker uses his tech - savvy skills.