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.
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.
In this fanfic, Cleo getting sick could lead to some interesting plot developments. Maybe it shows how the other characters in H20 react to her being ill. They might have to take care of her, which could strengthen their relationships.
They are popular because it shows Hermione in a vulnerable state. Usually, she's so strong and in control, but when she's sick, it's a new side of her.
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.
Well, if Naruto gets bitten instead of Sasuke, it could lead to a whole new set of powers for Naruto. Maybe he'd gain some sort of dark or unique ability that changes his fighting style. It could also change his relationship with other characters. For example, his friends might be more worried about him and try to find a way to cure him. And his enemies might see him as an even greater threat or a new target.
Sasuke could be sick of it because he values Naruto's strength and potential. He knows that Naruto is important for their village and for their future battles. Sakura hitting Naruto could potentially harm Naruto physically or mentally, which Sasuke doesn't want as it could affect their overall power balance and the future of the village.
A common scenario is when Naruto is given a special mission or task that Sasuke feels he should have been chosen for. Sasuke's pride and his competitive nature with Naruto make him jealous. He might think that Naruto is being favored over him.
In many Sasuke - gets - jealous - of - Naruto fanfictions, it often stems from Sasuke's insecurities. He may be jealous of Naruto's natural ability to make friends easily. Naruto has a charisma that draws people in, and Sasuke, who has always been more of an loner, may feel left out or envious. For instance, if Naruto is praised for his skills in a particular area, Sasuke might feel jealous as he also wants that kind of recognition but is too proud to show it in a normal way.
I'm not sure if there is such a specific fanfic. Fanfiction can cover a wide range of scenarios, but cheating themes might not be very common in the Naruto fanfic world, especially considering the strong bond between Sasuke and Naruto. However, with the vast amount of fanfiction out there, it's possible that one exists.