Another possibility is a new character created specifically for the fanfiction. This new character could have entered Naruto's life, and for some reason, things didn't work out, leading to the dumping. It would give the author more creative freedom to develop a unique 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.
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.
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.
In some 'naruto gets dumped fanfiction', there could be a plotline where Naruto's friends try to cheer him up. They might take him on adventures or introduce him to new people. This could show the importance of friendship during tough times and how his friends can help him get over the dumped experience.
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.
Sakura would be a likely candidate. As a skilled medical ninja, she has the knowledge and ability to diagnose the poison and attempt to create an antidote.
It could be Orochimaru. He's always interested in powerful artifacts, and the mask Naruto gets would be no exception. Orochimaru would likely try to steal the mask from Naruto to add to his collection of powerful items and use its power for his own dark experiments.
Sakura could potentially be Naruto's girlfriend in some fanfictions. Although in the original series she had feelings for Sasuke for a long time, in fan - created stories, the writers might explore a different path where she realizes Naruto's true worth and they end up together.
He could marry Mikasa from Attack on Titan. Just think about it. Naruto's positive and determined personality would contrast with Mikasa's more stoic and protective nature. They could meet through some sort of interdimensional accident. Naruto would be fascinated by the Titans and the world outside his own, and Mikasa might be intrigued by Naruto's ninja abilities.
He could marry Hinata. Even though he's banished, Hinata might follow him because of her deep love for him. Their relationship could develop further outside the village, and they could get married and start a family.
Maybe it's a completely new character introduced in the fanfiction. This new character could have some special powers or a unique connection to Naruto that results in the pregnancy.