Danny Phantom headcanon that says when Danny dies, he’ll turn fully into a ghost, and become a permanent resident in the Ghost Zone.
Unfortunately, he loses memory of who he was in his past life, but somehow the only thing that lives in him is his hero heart to save and protect everyone around him. He became the protector of the Ghost Zone, protecting it from any trouble or unwanted visitors, in this case, humans.
Because of this, when his family and friends decides to visit him, he either gives them a warning to NOT enter the Ghost Zone, or he attacks them furiously, REFUSING TO LISTEN TO ANY OF THEIR EXPLANATIONS.
Despite all this, they still try endlessly and constantly visit him everyday, in hopes to finally convince him that he was once human, and that he had a family.