Maybe it's an enemy from another village who sees Naruto as a threat to their plans. For example, if there's a group in another village that wants to start a war, they might think that getting rid of Naruto would give them an advantage. Another possibility is that it's a character who has been brainwashed or controlled by an evil force. This could add an interesting layer to the story as Naruto's friends would have to not only deal with his injury but also find a way to break the control on the stabber.
It could be someone from Naruto's past that he thought was a friend but has turned against him for some reason. This could be a person who has a grudge because they felt Naruto overshadowed them in some way, like in their ninja training or in the eyes of the village. Or it could be someone who was influenced by a powerful and malicious ninja who wants to see Naruto dead.
In 'naruto gets stabbed fanfiction', it might be a new enemy that has emerged, someone with a powerful and unknown jutsu. This enemy could see Naruto as a threat to their plans and decide to stab him to eliminate him. Or it could be a character who has been corrupted by dark power, like a former friend or ally who has fallen to the dark side and no longer recognizes Naruto as a friend.
It could be an enemy ninja from another village who wants to take out Konoha's strongest shinobi. Maybe someone from the Sound Village still holding a grudge against Naruto.
Another possibility is that it could be a cursed or possessed person. They might not be in control of their actions and end up biting Naruto. This could be part of a larger, darker plot where a curse is spreading and Naruto becomes an accidental victim instead of Sasuke.
Another possibility could be a new character introduced in the fanfic. This new character might be using Naruto to gain something, like power or influence in the ninja world, and when they get what they want, they cheat on him. It would add an element of mystery and betrayal to the story.
It could be Sakura. In the original series, their relationship was complex, and in fanfiction, it would be a common choice for her to be the one to dump Naruto, perhaps due to her still having feelings for Sasuke.
Another candidate could be Kakashi. As Naruto's sensei, he is in charge of Naruto's training and development. If Naruto continuously disobeys orders or fails to progress as he should, Kakashi might use this unorthodox method of spanking to get Naruto back on track.
It could be Orochimaru. He has always been interested in Naruto's power and potential. He might kidnap Naruto to try and extract the Nine - Tails' chakra or study his unique Uzumaki bloodline further. Another possibility is a group of rogue ninjas who want to sell Naruto to the highest bidder, thinking his value as a Jinchuriki would make them rich.
Maybe Kakashi could still be his mentor. Even though One for All is a new and different power, Kakashi is very experienced. He could help Naruto understand how to incorporate this new power into his overall combat strategy. Kakashi would be able to analyze the power and give Naruto advice on how to train with it while still relying on his ninja skills.
It could be a powerful enemy like Orochimaru. Orochimaru was always interested in Sasuke's potential, but if the plot changes and Naruto gets bitten, Orochimaru might see Naruto as an equally interesting subject to experiment on.
Maybe he marries an OC (original character). The author of the fanfic could create a whole new character with unique qualities that attract Naruto during his time away from Konoha.