Definitely Danny's kids if he has any in the fanfiction universe. They could inherit his powers and have to learn how to use them while dealing with their own personal issues. Also, Sam and Tucker's children could be main characters. They might not have ghost powers but could be very tech - savvy and help in other ways, like creating new gadgets to fight ghosts.
Another possibility is that an old enemy of Danny's has a child who is not as evil as their parent. This character could be conflicted between their family's history and their new - found friendship with the next - generation heroes. They could be a main character as they try to bridge the gap between the two worlds of ghosts and humans.
Maybe some new characters who are students at Casper High. They could be inspired by the stories of Danny Phantom and then suddenly find themselves in the middle of a ghost - related crisis. One of them could be a loner who discovers they have a special connection to the Ghost Zone, and this connection makes them a target for both good and bad ghosts.
There could be a character like Nova, a young girl who has the ability to phase through solid objects like a ghost but is human. She uses this power to help the new generation of ghost hunters.
Danny's parents could be main characters too. They might be trying to throw Danny a normal human birthday party, completely unaware of the ghostly happenings that often surround their son. Their attempts to have a 'normal' celebration could be constantly interrupted by Danny having to slip away to deal with ghost problems. This would create an interesting dynamic between Danny trying to keep his double life a secret from his parents while still enjoying his birthday with them.
The children of Rachel and Finn could be main characters. Maybe their son or daughter inherits Rachel's amazing singing talent and Finn's charm. They could struggle with living up to their parents' legacy in the glee club.
Definitely the children of Ladybug and Chat Noir. They would inherit the Miraculous and have big shoes to fill. Their personalities could be a mix of their parents' traits, like the daughter being as brave as Ladybug and having Chat Noir's sense of humor.
There could also be a character who is not directly related to the original ninja but has a special connection to the elemental powers. Maybe a young girl who discovers she has an ability to control an element that was previously thought to be lost. She would bring a fresh perspective to the group of the next generation and have to prove herself to the others.
One possible main character could be the son or daughter of Iron Man. They might inherit his genius and some of his tech - savvy ways. They could also have a bit of his cocky attitude at the start but grow into a true hero throughout the story.
The main character could be a person who was once a part of an evil organization but has had a change of heart. In the 'pokemon next generation fanfiction', they use their knowledge of the organization's ways to protect the new generation of Pokemon. They start with a single Pokemon that they rescued from the organization and build a team as they go, facing challenges from their former comrades and also making new friends among other trainers.
The proteges of the Avengers could be main characters too. Maybe Hawkeye has been training a young archer who has amazing accuracy but lacks confidence. This protege could be part of the next - generation Avengers and grow throughout the fanfiction, facing challenges that force them to become more self - assured.
New students from different kingdoms could also be main characters. For instance, a student from Vacuo who has a unique sand - based fighting style. Or a student from Atlas who is a tech genius and uses advanced weapons. They could team up with the descendants of Team RWBY to form a new, diverse team.
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.