Well, perhaps the plot involves Harry getting into all sorts of comical misunderstandings. For example, he could mistake the host club's activities for some kind of magical training. And then he tries to use his wizard skills to fit in. He might also get involved in a love triangle with one of the male hosts and a female student who is smitten with Harry's mysterious aura.
Maybe Haruhi. Haruhi is down - to - earth and not as caught up in the extravagance of the school as some others. Harry, coming from a more humble background in the wizarding world (compared to the wealth of Ouran), would relate to Haruhi. They could become friends as they both have a bit of an outsider perspective within the school environment.
In the fanfiction, Naruto could end up joining some of the clubs. Maybe he'd be interested in the Host Club because he likes making friends and being around people. But his straightforward and sometimes brash personality might clash with the more refined and proper behavior expected in the club. He could bring a whole new energy to the club, though, with his determination and enthusiasm.
Another plot could be that Harry is mistaken for someone else at Ouran. He then has to navigate through the misunderstandings while also trying to figure out how the Ouran world, with its own set of rules about class, social status, and relationships, compares to his world at Hogwarts. There could be a lot of comical moments as he tries to fit in without revealing his true identity as a wizard.
There might be a character who is secretly from the wizarding world but has turned to the dark side. This character could be after Harry in the Ouran setting, trying to use the chaos of the new environment to their advantage to get to Harry without being noticed by the Hogwarts professors.
A fanfic with this scenario could start with Harry arriving at Ouran Highschool on a rainy day. He has his trusty umbrella, which could be a family heirloom or a magical item. As he walks through the school grounds, the students of Ouran Highschool are intrigued by his different appearance and the old - fashioned umbrella.
His bright orange jumpsuit would definitely make him stand out. In a school full of students wearing traditional school uniforms, Naruto's unique ninja attire would draw a lot of attention. His spiky blond hair would also be quite different from the normal hairstyles at Ouran Highschool.
One possible plot for such a fanfic could be that on a rainy day at Ouran Highschool, a character similar to Harry (maybe with some of his traits) shows up with a special umbrella. The umbrella might have some magical powers related to the rain, like creating a rain - free zone around the holder. And this character could interact with the Host Club members in unique ways because of this umbrella.
Romance is often a theme as well. With the Host Club's focus on charming and wooing guests, it's natural that there are romantic elements when combined with the Harry Potter characters. There could be love triangles or unexpected pairings that add a lot of drama and intrigue to the fanfictions.
A 'what - if' scenario where Haruhi is actually from a wealthy family but has been hiding it all along. This would add a new layer of mystery and drama to the relationships within the host club.
In a fanfiction where Naruto goes to Highschool DxD, he could introduce new concepts of chakra to the world. This would be really fascinating as the characters in DxD might try to incorporate it into their own powers. Naruto could also be a bit of a fish out of water, not fully understanding the social norms and hierarchies in Highschool DxD at first. But his kind heart would help him navigate through it all, and he'd end up changing the school environment in a positive way with his unique personality.