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.
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.
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.
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.
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.
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, inexperienced doctor. Maybe they make a mistake during a surgery that Meredith is involved in, accidentally causing her harm. For example, they might misread a vital sign or use the wrong instrument.
It could be a rival assassin. They might see Kassandra as a threat to their own reputation or contracts.
It could be a Sith or a Sith - inspired enemy. They are often the ones causing trouble for the Jedi, and shooting Ahsoka would be a way to strike at the Jedi Order.
There's also a possibility that it's an accident. In the chaos of a battle or a tense situation, a stray bullet from either side could hit Katniss, even if no one actually intended to harm her directly.