There could be fear as well. Seeing Naruto injured, Sakura might be afraid of losing him. This fear could drive her actions in the fanfiction, whether it's in finding a cure for his injuries or in taking revenge on those who hurt him. It would also add more depth to their relationship as she grapples with these complex emotions.
One common plotline could be Naruto facing a powerful enemy alone to safeguard Sakura. For example, in a fight against a rogue ninja who has a vendetta against Sakura's medical skills, Naruto jumps in front of an attack meant for her. This could lead to him getting seriously injured and Sakura having to use her medical ninjutsu to save him while also realizing her true feelings for Naruto.
It could be due to Sakura's feelings for Sasuke. If she's overly focused on Sasuke and Naruto gets in the way of her plans regarding Sasuke in some way, she might lash out and hurt Naruto without really meaning to cause serious harm.
Sakura would be devastated. She would probably blame herself for Naruto's death since he died protecting her. She might go into a state of deep depression at first, withdrawing from everyone.
One possible fanfiction plot could be during a village invasion. Naruto, with his unwavering sense of justice and his growing feelings for Hinata, shields her from an enemy's attack. He might get seriously injured, which then spurs Hinata to tap into her own hidden power to defend him in return. This kind of story can explore their bond deeper and show how they support each other in tough times.
There could be a fanfiction where Naruto and Ino are exploring an ancient ruin. Unbeknownst to them, there are traps everywhere. When a huge boulder starts rolling towards Ino, Naruto pushes her out of the way. He gets crushed under the boulder's edge. Ino is desperate to save him. She manages to find some herbs in the ruin that have healing properties and uses her medical knowledge to treat Naruto's injuries. Meanwhile, Naruto's willpower also helps him in his recovery.
In a hurt comfort fic, perhaps Sakura has an emotional breakdown after a failed mission. Naruto finds her crying alone. He sits beside her and just listens as she pours out her frustrations. Naruto then shares his own insecurities, like how he sometimes doubts if he can really become Hokage. This mutual sharing of pain and comfort brings them closer, strengthening their friendship in a way that goes beyond their ninja duties.
In some fanfictions, Sakura might accidentally hurt Naruto during a training session. Maybe she uses a new jutsu she's not fully mastered and it goes wrong, hitting Naruto instead of the target.
One common theme is misunderstanding. Often, Naruto may misunderstand Sakura's intentions or vice versa, which leads to one of them getting hurt. For example, Naruto might misinterpret Sakura's concern for him as pity, and his reaction could hurt her.
Well, in some Narusaku fanfictions, Sakura might hurt Naruto emotionally by choosing Sasuke over him repeatedly. This can make Naruto feel rejected and unloved, causing him pain.