Perhaps in the fanfiction, Danny is betrayed by a new ally he trusted. This new character could have joined Danny in his fight against evil ghosts, but in reality, they were working for a more powerful and evil ghost all along. They might use Danny's trust to learn about his weaknesses and then expose him to danger, leaving Danny feeling hurt and alone, having to reevaluate who he can trust in the future.
One possible scenario could be that Sam betrays Danny's secret identity to someone he doesn't trust, like a ghost hunter. Maybe she does it out of jealousy if Danny starts getting closer to another character.
One possible scenario could be Danny getting injured during a fight with a powerful ghost. Maybe he underestimated the ghost's new power and got hit by a strong energy blast that left him with a serious injury.
One possible scenario could be Danny getting bullied at school. Maybe some of the other students find out about his ghost powers and they target him out of fear or jealousy. Another could be that he gets injured during a ghost attack that takes place on school grounds while he's trying to protect his classmates.
In some fanfictions, it could happen during a moment of vulnerability. For example, if Sam gets hurt while they're on an adventure, and Danny is really worried about her. As she recovers, Danny realizes how much she means to him and they share a kiss. Or maybe they get stuck in a ghost - created alternate dimension where they think they might not get out. In that desperate situation, they confess their feelings and kiss for the first time.
One scenario could be when Danny is in a fight with a villain who has heat - based powers. The intense heat generated by the villain could be too much for Danny to handle, causing him pain and weakening his powers. Another might be if he's trapped in a place with extreme heat, like a factory furnace in the Ghost Zone, and he struggles to escape while the heat takes a toll on him.
In some fanfictions, a massage could be part of a spa day for the characters. For example, Sam, Tucker, and Danny could all go to a ghost - friendly spa (in a fan - made concept) and there could be humorous situations during the massages. Maybe Danny's powers go a bit haywire during the relaxation process. This type of scenario is great for adding some light - heartedness to the story.
One idea could be that Danny's friends turn on him because they believe false rumors spread by a new villain in town. Maybe the villain makes it look like Danny is using his powers for evil. Another could be that the government agency that was once somewhat friendly to Danny decides he's too dangerous and betrays his trust by trying to capture him.
One possible scenario could be a character being betrayed by a teammate. For example, Ruby might be betrayed by Weiss if Weiss is lured to the dark side by some powerful and evil force. This could lead to intense battles and emotional turmoil for Ruby.
One interesting scenario could be when they share their first kiss during a dangerous ghost - fighting situation. Maybe they think they won't make it out alive and finally confess their feelings through that kiss.
Well, in some 'Danny Phantom Betrayed' crossovers, Danny might be betrayed by a love interest. For example, if it's a crossover with another show and he falls for a character from that show, but that character betrays him for their own gain or because of some hidden agenda. It could lead to Danny having to reevaluate his trust in people and also face new challenges that come with the heartbreak and the betrayal. Also, he could be betrayed by an authority figure in a crossover universe, like a powerful wizard or a superhero leader who turns out to have ulterior motives.