In some fanfictions, an ancient evil force or a powerful demon might be the cause. They might be awakened or released somehow and Naruto, being one of the strongest in the village, is the first to face it. But this powerful entity's strength is overwhelming for Naruto, leading to his bad injury.
A new and powerful rogue ninja could be responsible. This ninja might have a vendetta against Konoha and see Naruto as a symbol of the village, so they target him and use their strong and devious techniques to injure him badly.
The Riddler could be responsible. He might set up a series of elaborate and deadly traps throughout Gotham. Batman, in his attempt to solve the riddles and stop the traps from harming innocent people, could end up getting caught in one of them and getting hurt badly. For instance, a trap could involve a collapsing building or a room filled with sharp objects.
Another possibility is that it's a rogue Shadowhunter. There are always those in the community who don't agree with the way things are going. If Clary's actions or ideas are seen as too radical, this rogue Shadowhunter might try to stop her by injuring her. This could also lead to a story about Clary trying to understand the motives of this attacker and potentially reform them.
It could be an enemy. For example, a rival in a competition or a villain who has a grudge against Leo.
It could be a new and powerful villain who has emerged. This villain might have a special ability that catches Naruto off - guard during a fight, leading to his injury.
Another scenario could be in a mission gone wrong. The fanfiction might depict a mission where Naruto and his team are caught off - guard by an ambush. There could be a large number of well - coordinated enemies, and Naruto might end up being the main target, getting severely injured as he tries to fight them off and protect his team.
The Projectionist might be responsible. As the Projectionist is always lurking around in the dark corners of the studio, it could mistake Bendy for an intruder and attack him. Its large and menacing form could easily cause some serious injuries to Bendy.
One common plot development could be his comrades rallying around him. For example, Naruto might be determined to find a way to heal him quickly, using his newfound powers or connections. Another could be Kakashi reflecting on his life while injured, thinking about his past mistakes and how he can grow from them if he recovers. Maybe he also has some sort of vision or encounter with a long - lost friend or relative in his weakened state.
It might be that he was trying to protect some of the Fairy Tail guild members from a dangerous enemy. In the process, he took on more damage than he could handle. For example, if a powerful demon was attacking the guild, Naruto might jump in front of it to save his newfound friends, getting seriously injured in the process.
One possible fanfiction could be that Naruto is transported to the Fairy Tail world through some magical anomaly. In a battle against a powerful dark mage in Fairy Tail, Naruto, not fully understanding the magic system there, gets badly injured. He has to rely on the kindness of the Fairy Tail guild members to heal and learn about their magic to get back on his feet.
A rogue ninja from a rival village could be responsible. They might see Naruto as a threat and use their fire - based attacks to harm him. For instance, a ninja from the Hidden Mist Village who has a grudge against Konoha and targets Naruto specifically.