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.
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 during a battle at Hogwarts. Maybe the Death Eaters target her specifically, and she gets hit by a dark curse while trying to protect her friends. Another could be in an accident in the Forbidden Forest. She might be exploring and stumble upon some dangerous magical creatures, like a centaur gone wild, and get injured in the encounter.
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 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.
In the 'tmnt leo gets injured fanfiction', one possible scenario could be that Leo gets injured during a fight with the Foot Clan. Maybe he underestimates their new weaponry and gets a deep cut on his arm. Another could be that he's injured while trying to protect his brothers from a falling building, getting his leg trapped under rubble.
One possible plotline could be Danny trying to hide his sickness from his enemies while still fighting ghosts. He might be weaker than usual, but his determination keeps him going.
Maybe it's one of his old enemies who has found a new way to attack him. For instance, Vlad could come up with a more devious plan that manages to catch Danny off - guard and injure him. Vlad has always been a formidable foe, and he might use Danny's overconfidence or a moment of weakness against him.
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.
One popular Danny Phantom injured fanfiction might be 'The Battle's Aftermath'. It focuses on Danny's recovery after a particularly tough fight with a powerful ghost. The author does a great job of showing his physical and emotional pain, and how his friends and family support him through it.