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 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 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.
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.
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.
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.
His ghost - hunting parents could potentially betray him. If they found out Danny was half - ghost, they might see him as a threat to humanity rather than their son. They've been so dedicated to hunting ghosts that their views could be clouded, and they might not be able to accept Danny for what he is, leading to a heartbreaking betrayal for Danny.
It could be Valerie. She has had a somewhat rocky relationship with Danny in the original series. In a fanfiction about betrayal, she might be pushed over the edge by jealousy or a misunderstanding about Danny's powers and his intentions. She could then use her ghost - hunting skills against Danny in a more malicious way than before.
He could start to lose strength. Food provides the energy needed for normal human activities, and even though Danny has ghost powers, his human side still needs proper nourishment. Without eating, he might find it harder to fight ghosts effectively as his physical strength wanes.