Yes, there might be such fanfics in the vast Naruto fanfic community. However, these often deviate from the original story's canon. Fanfics are a creative space for fans to explore different scenarios, but they can vary greatly in quality and adherence to the characters' original personalities.
One idea could be that Sasuke, despite his usually stoic nature, feels a sense of worry when he hears Naruto is sick on his birthday. He could visit Naruto and bring him some traditional Uchiha herbal medicine that he secretly prepared. This would show a soft side of Sasuke and also deepen their bond in the fanfic.
Sasuke could play a crucial role. He might use his Sharingan to detect what's wrong with Naruto more accurately. Then he could go on a mission to find a special cure.
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.
The fanfic could explore how Sasuke deals with his emotions when Naruto is sick. He might be conflicted between his old rivalry feelings and his newfound, deeper feelings for Naruto. Sasuke could also be the one to stay by Naruto's side when everyone else is busy. He'd watch over Naruto while he sleeps, and when Naruto wakes up, there could be a moment of intense eye - contact that shows the unspoken bond between them.
In such a fanfic, Naruto might gain some unique abilities from the bite. Maybe it would change his chakra nature or give him a new form. It could also lead to a different relationship with the creature that bit him. For example, if it was a demon, he might have to deal with its influence in a new way compared to Sasuke.
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.
One possible theme could be the exploration of an alternate universe where the laws of biology are different. Another might be a deep exploration of their relationship, showing a level of intimacy and connection that goes beyond the normal friendship or rivalry in the original series.
Maybe in the fanfic, a dark force is trying to corrupt Sasuke's Sharingan. Sasuke, realizing he can't control it on his own, gives it to Naruto. Naruto, with his strong will, battles the dark influence within the Sharingan. He uses his own positive energy and friendship bonds to purify the Sharingan. Eventually, he masters it and uses it to bring light back to the ninja world, also using it to reunite with Sasuke in a more powerful and united way.
Naruto's character would change in multiple aspects. First, he would have to adapt to the new abilities of the Rinnegan. This would require him to study and train more, making him more focused and perhaps a bit less impulsive. Second, the power of the Rinnegan might give him a different perspective on the ninja world. He might start to see the power struggles and politics in a new light. Third, his relationships with his friends, especially Sasuke, would be different. Since he now has a part of Sasuke (in the form of the Rinnegan), their bond could become even more complex and deep. And finally, he might face new challenges in terms of his identity, as he is now not just the Nine - Tails Jinchuriki Naruto, but also the wielder of the Rinnegan.