Yes, in some fanfics, Harry Potter could be reimagined as a Jounin. Fanfic writers often like to cross different fictional universes and concepts, so it's possible to see Harry with the abilities or status of a Jounin from another series in their creative works.
He would probably be very powerful. With his existing magical abilities from the Harry Potter world combined with the skills of a Jounin, he could be an unbeatable force. He might use magic in ways similar to how a Jounin uses their combat and strategic skills.
Well, just based on that title alone, it's difficult to determine its quality. There are so many aspects that make a Harry Potter fanfic great. For example, it could explore new relationships between the characters, or create exciting new magical adventures. But we don't know if 'skull is harry potter fanfic' does any of those things. So, it might be good, it might not.
Maybe he would have mastered some new wind - style jutsus. For example, he could create more complex and powerful wind blades that can cut through even stronger defenses. Since he is at the Jounin level, his chakra control would be much more refined, enabling him to perform these advanced techniques.
Well, Asuma is also a character that shows up a lot in these fanfics. His relationship with his students and his smoking habit give fanfic writers a lot to work with. And then there's Kurenai. Her genjutsu skills and her role as a female jounin can add unique elements to the stories. There are also some original characters created by fanfic authors that sometimes become popular within the 'naruto jounin fanfic' community.
I'm not sure if it's common. There are so many different types of fanfics in the Harry Potter fandom. Some focus on relationships, others on new adventures. While metamorph - related ideas are interesting, it might not be as common as, say, romance - centered fanfics.
Kakashi could still be an important ally even though Naruto has become a Jounin. Kakashi has a wealth of experience and knowledge that Naruto could still learn from. He could offer advice and assistance when Naruto faces tough challenges in his new role.
In this kind of fanfic, one interesting storyline could be Naruto facing new and more powerful enemies as a Jounin. He might have to use his unique skills in more strategic ways. For example, his shadow clone jutsu could be used to deceive opponents on a much larger scale.
One interesting plot could be a jounin who has a secret mission to infiltrate an enemy village. He has to use his unique jutsu and skills to avoid detection. Another plot might involve a jounin training a group of genin in a very unorthodox way that challenges the traditional shinobi training methods.
One common theme is self - discovery. Since backup jounins are often in the background, these fanfics usually explore how they find their own strength and purpose. Another theme is loyalty. They have to show loyalty to their village and comrades even when they are not in the spotlight.