Another scenario could be that Danny is getting ready for a special event. He's in his room and realizes that the shirt he's wearing doesn't quite match the look he's going for. So, he takes it off to change into something more suitable. This could lead to some interesting interactions with his friends or family who might walk in on him while he's changing, creating some comical or heart - warming moments.
One possible scenario could be that Naruto is about to engage in a tough training session. He takes his shirt off to feel more unrestricted and ready to push his limits. It shows his determination and eagerness to become stronger.
A reason could be that he's overheating. Danny's powers might sometimes cause his body temperature to rise, and taking off his shirt helps him cool down. It's a simple practical reason that could be used in a fanfiction to show his more human side, dealing with the physical discomforts that come with his ghostly powers.
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.
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 is when his parents accidentally harm one of his ghost friends while trying to hunt ghosts. Danny might get really angry and show his hatred towards them for being so reckless. Another scenario could be during a family dinner when they start talking about their latest ghost - hunting inventions and how they plan to use them to catch more ghosts. Danny, feeling threatened by this, could snap and show his dislike for them.
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.
One possible scenario could be that Percy got injured and Annabeth needs to tend to his wounds on his torso, so she takes his shirt off. For example, he might have been scratched by a monster during a battle, and she has to clean and bandage the wound quickly.
It could also be that Superman underestimates Danny's powers. Danny has his own set of unique abilities as a half - ghost. When Superman dismisses him or tries to boss him around, Danny stands up for himself and tells Superman off. He might show Superman some of his cool ghost powers like intangibility and invisibility in a rather sassy way to prove that he's not someone to be pushed around.