One of the most popular crossover scenarios might be the ones that involve unique power combinations. For example, in a Naruto and Highschool DxD crossover, Naruto's jutsu combined with the devil powers in Highschool DxD could create really interesting battles. In a Rosario Vampire and Highschool DxD crossover, the different types of supernatural beings from both series could interact in new and exciting ways. Also, crossovers that explore the different school settings and how the characters from each series adapt to them are quite popular.
Probably the crossovers that focus on character relationships are very popular. Imagine Naruto's friendship values clashing or meshing with the more complex relationship dynamics in Highschool DxD. Or in a Rosario Vampire and Highschool DxD crossover, the way vampires interact with devils. It's all about the drama and the new interactions that these crossovers can bring.
I'm not sure if there is such a specific fanfiction where a character from Rosario Vampire is reading Naruto fanfiction. Fanfiction can be very diverse, but this seems like a rather unique and specific concept. It's possible that it might exist in the vast realm of fan - created stories, but it would be quite difficult to find without a more targeted search.
A really cool crossover idea would be a tournament - style plot where characters from both Highschool DxD and Naruto compete. This would showcase the different fighting styles and abilities. The ninjas' stealth and teamwork could be pitted against the devils' and angels' supernatural powers. It would be a great way to explore how the two very different universes' characters would react to each other in a competitive environment.
In these fanfictions, the power systems can interact in some really cool ways. Naruto's chakra system is very different from the devil powers in Highschool DxD. But when combined, you could have a situation where Naruto's chakra is used to enhance a devil's attack. In Rosario Vampire, the vampire powers are more about physical abilities and some magic. They could interact with Naruto's ninjutsu in a way that creates new forms of combat, like a vampire using Naruto's shadow clone jutsu to create multiple decoys.
One popular concept is the power exchange. For example, Naruto's unique jutsu skills being introduced to the Highschool DxD world. Maybe a character from Highschool DxD learning the Rasengan. Another concept could be the character relationships. How would Naruto interact with Issei and his friends? They might form an unlikely alliance against common enemies. Also, the idea of combining the different mythologies in both series is fascinating. The shinobi world's gods and the devils and angels from Highschool DxD could create some really interesting storylines.
Rias from Highschool DxD is likely to be a popular choice in crossovers. Her status as a powerful devil and her leadership role can make for some exciting scenarios when combined with the Persona 3 universe. Another popular character could be Aigis from Persona 3. Her robotic nature and combat abilities can be quite interesting when pitted against the supernatural beings in Highschool DxD.
A less common but still interesting trope is the 'reversal of roles'. In some fanfictions, Issei might find himself in a world where his devil powers are seen as something different or even feared in a new way. Maybe in a world of pure angels, he is the outcast and has to prove himself not to be an evil entity. This trope allows for a lot of character exploration and development for Issei.
Issei Hyoudou is often a very popular character in these crossovers. His ability to transform and his harem - like relationships make him an interesting element to bring into other fictional universes. Rias Gremory is also quite popular. As a powerful devil, she can hold her own in crossovers, whether it's against other powerful characters from different animes or in new and strange situations. Akeno Himejima is another one. Her unique personality and powers make for great interactions in crossover scenarios.
Naruto himself is often very popular. His energetic and determined personality makes him stand out. He can bring his ninja skills and positive attitude into the Highschool DxD setting. Also, Issei from Highschool DxD is commonly included because of his central role in his own series and his unique relationship with the devils and other supernatural beings.
One possible fanfiction could be about Naruto somehow ending up in the world of Highschool DxD and meeting Moka. Maybe Naruto's skills and his never - give - up attitude would impress Moka and the other characters. He could teach them some of his ninja techniques, like the Shadow Clone Jutsu, which could be useful in battles against the devils and other supernatural beings in the Highschool DxD world.
Naruto and Issei are very likely crossover characters. Naruto, with his strong will and powerful ninja skills, would be an interesting contrast to Issei, the perverted but also determined protagonist of Highschool DxD. Another likely pair could be Sasuke and Vali. Both are brooding, powerful characters with their own agendas. Sasuke's Sharingan abilities could have some really cool interactions with Vali's powers in the context of this fanfiction. And of course, Kakashi could be a great crossover with some of the more strategic characters in Highschool DxD, like Sona, as he is known for his intelligence and combat skills.