Some fans would be really shocked and sad. Kakashi is a very popular character, and his death at the hands of Obito would be a huge blow to many. They might feel angry towards the author for creating such a tragic scenario.
Sakura would probably be very shocked at first. She might be worried about Naruto's well - being and also a bit fearful of the combined entity. Kakashi, on the other hand, would be more understanding. Given his connection to both Naruto and Obito, he would try to communicate with the merged form to figure out what happened. Sasuke would be initially suspicious, thinking it could be some new threat. But after seeing that Naruto's essence is still there, he might be more accepting and see it as a new power to use against their common enemies.
Yes, there are likely many fanfics out there with the concept of Naruto being at Jonin level. You can check fanfic platforms like Fanfiction.net or Archive of Our Own.
Sakura may also use her medical ninjutsu in a creative way. For instance, she could try to numb the part of Obito's body that is restraining her by using her precise chakra control. Or she might use some kind of medical jutsu to create a distraction, like a bright flash of light or a small explosion of medical energy.
One great young Jonin Naruto fanfic is 'The Ascent of a Young Jonin'. It focuses on Naruto's journey as he transitions into a Jonin, facing new challenges and forming deeper relationships with his friends. The story has some really exciting battle scenes and character development.
New storylines. Fanfics can create unique scenarios. Maybe Naruto has to team up with unlikely allies as a young Jonin to face a powerful enemy that the village has never seen before. It adds an element of excitement and unpredictability.
One possible plot could be Naruto facing new and tougher challenges as a young Jonin. He might be assigned to lead a special mission that requires him to use his unique skills and leadership abilities. For example, he could be sent to a border area where there are constant skirmishes with enemy ninja. Naruto would have to strategize and protect his team members, all while dealing with the expectations that come with being a Jonin.
Sasuke could also be a main character in the 'naruto jonin exams fanfic'. His skills and his rivalry with Naruto would make for an interesting plotline during the exams. Also, Sakura might be a main character as well. She has grown as a ninja and could show off her new abilities during the jonin exams.