You could also create a story where there is a common enemy for both Naruto and Monster High. This enemy could be threatening both the ninja world and the world of Monster High. So, Naruto and the Monster High gang have to team up. The Monster High students could use their unique powers, and Naruto and his friends could use their ninja techniques to fight this common foe.
Another approach is to mix the power systems. In fanfiction, the chakra from Naruto could be related to the power of the devils or other beings in High School DxD. For instance, the tailed - beast chakra could be equivalent to a powerful devil's power source. Characters from both universes could then try to learn about and utilize the other's power system. This would create a lot of interesting scenarios where they have to train together or compete to see who has the stronger power.
One way to combine them could be by having Naruto characters enter the world of High School DxD. For example, Naruto with his determination and unique abilities could interact with the devils and other supernatural beings in the DxD world. Maybe he'd form an unlikely alliance or even compete in the Rating Games.
One way is through the concept of power transfer. For instance, a character from Naruto could accidentally gain the power of a devil in High School DxD and have to figure out how to control it. This would lead to some really cool action scenes as they struggle with the new power. Another way is through cultural exchange. The shinobi culture from Naruto could be introduced to the High School DxD characters, creating some interesting interactions.
They can be combined by having a shared plot device. Maybe a powerful artifact that is important in both Naruto's world and the High School DxD world. This could bring the characters from both series together. The ninjas could be interested in the artifact for its power to protect their village, while the devils and angels in High School DxD might want it for their own purposes like increasing their power or influence. It could also lead to some characters from each series having to team up to protect or obtain the artifact.
Perhaps a crossover where the shinobi world and the Highschool DxD world are somehow connected. Maybe there's a portal that leads from Konoha to the school. This would allow for all kinds of interactions between the characters from both universes, like Sasuke getting involved in the power struggles in the Highschool DxD world.
A possible combination could involve a shared goal. Say there is a powerful and ancient treasure that has the power to change the balance of all three universes. Naruto characters, with their determination and unique abilities, CP9 with their stealth and combat skills, and One Piece characters with their nautical knowledge and diverse powers, all come together to search for this treasure while facing various obstacles and enemies along the way.
Another possibility is to have the characters from the Karakuri Circus be transplanted into the Naruto world. They would have to adapt to the ninja society. The Karakuri puppets could be seen as a new type of weapon or tool in the Naruto world. And the circus performers could use their unique skills to either help or cause trouble in the ninja villages.
Another way is to have Naruto fuse his chakra with someone who has the ability to use Susanoo, like Sasuke. They could create a combined form that has the best of both their powers. The story could detail their journey of learning to work together and master this new form.
One interesting Naruto Monster High fanfiction could be where Naruto attends Monster High as an exchange student. He might show off his ninja skills during gym class or make friends with the unique monster students. Maybe he'd have a friendly rivalry with Draculaura in a race.
One way to combine them could be in a crossover universe. For example, Naruto could be summoned to the world of Highschool DxD through some sort of magical mishap. Gabriel, being an angel in her own right, could be involved in a plot to either help Naruto return home or to use his power for some greater purpose in the Highschool DxD world. Maybe Naruto's unique abilities, like his shadow clones or rasengan, could be seen as strange and powerful in the DxD world, and Gabriel might have insights on how to control or enhance them.