Danny could be really devastated at first. His self - esteem might take a huge hit. However, he has always been a resilient character. He could turn to his family for support, especially Jazz who has always been there for him. He might also explore the ghost zone more on his own, looking for answers about his powers or maybe even a way to get his friends back. In the process, he could learn new things about himself and his powers, which would help him grow into a more independent and powerful hero.
Well, Danny may first go through a stage of denial. He might think it's some sort of misunderstanding and try to reach out to his friends. When he realizes they really have abandoned him, he could get angry. This anger might fuel his ghost - fighting abilities even more. He might start to see the people in the town as his new friends in a way, as he protects them from ghosts. He could also meet new characters during his solo adventures who help him see that he is capable on his own.
Danny might initially feel a great deal of sadness and confusion. He could start to isolate himself even more, thinking that if his friends could abandon him, then others might as well. But as time goes on, he might realize that he doesn't need them to be strong. He could focus on his powers and use them to protect the town on his own, finding a new sense of purpose that doesn't rely on his friends.
In the fanfiction, it could be that Danny's friends have their own problems to deal with. They might think that being associated with Danny is bringing more trouble into their lives. Maybe they get threatened by a new villain who warns them to stay away from Danny. Or they could simply be growing apart as they enter different phases of their lives and their interests no longer align with Danny's.
Danny would probably be extremely hurt. He might initially go into denial, not wanting to believe that his friends could do such a thing to him. He'd keep trying to reach out to them, hoping it was all a misunderstanding.
Danny might react with panic at first. After all, he has been hiding his identity from his parents for so long. He could try to escape from them, using his ghost powers in a desperate attempt to get away before they can do anything drastic.
In Danny Phantom fanfiction, heat might affect Danny in various ways. For example, his ghost powers could be weakened as some fanfics might play with the idea that extreme conditions like heat disrupt the balance of his human - ghost hybrid state. It could also make it harder for him to stay invisible or intangible as his energy is being sapped by the heat.
In some fanfictions, Danny's mom might find out when she accidentally witnesses Danny transforming into his ghost form. Maybe she walks in on him at a crucial moment.
Danny Phantom might adjust by using his powers in new ways. For example, he could use his invisibility to help Catwoman during her heists or surveillance. His ghostly powers could give them an edge in Gotham's dangerous underworld. He may also learn new skills from Catwoman, like combat techniques and stealth. Over time, he'd start to see Catwoman as a new family member and adapt to the different rules and lifestyle of living with her.
Sam would probably be the first to be curious and excited. She might start researching about any possible connection between ghosts and dragons right away. Tucker would be a bit more cautious at first, but also amazed. He might be worried about how Danny can control this new form and if it will put him in more danger. Jazz would initially be shocked but then would try to be very supportive. She might offer to help Danny figure out the psychological aspects of having this new form and how it affects his identity as a half - ghost.
Well, in some versions, Danny might use his ghost powers to track Vlad's lair. He could turn invisible and sneak in, then overpower Vlad using his unique abilities like intangibility and ectoblasts to free his parents.
Danny's relationships change drastically. He distances himself from his friends and family. He no longer values their opinions or support. Instead, he might see them as obstacles or even threats. His friendships with Sam and Tucker are strained as he starts to engage in activities that they find morally wrong, like using excessive force on other ghosts or humans who get in his way.
Sam would probably be really intrigued. She might see it as a cool new form for Danny and would be eager to learn more about how it affects his powers. Tucker, on the other hand, would likely be more concerned about how to reverse it at first. But once he realizes that Danny can still do some cool things as a mermaid, he'd be excited too.