Yes, there are many. You can find them on various fanfiction websites. Some might focus on crossovers between the two universes, like having characters from Dragon Ball attend the high school in DxD. Others could explore unique power combinations or relationships between the characters from both series.
One common theme is power - up. Combining the power - up mechanics from Dragon Ball with the devil - related power growth in High School DxD. Another theme is friendship, as characters from both series form new bonds.
You can try some popular fanfiction websites like FanFiction.net or Archive of Our Own (AO3). They usually have a large collection of fanfictions including those related to Dragon Ball Z and High School DxD.
Another way the storylines could merge is through the exploration of different forms of power. In Dragon Ball Super, there are various forms like Super Saiyan Blue, Ultra Instinct. In High School DxD, there are Sacred Gears and Devil powers. The characters could start to experiment with combining these powers or learning from each other about how to better utilize their own powers. For instance, Vegeta might be interested in how the Devils in High School DxD use their power in a more strategic way compared to his straightforward combat style in Dragon Ball Super. This could lead to character development and new plot twists as they adapt to each other's power systems.
One interesting crossover could be the power-ups. For example, Issei from High School DxD could gain some of the Saiyan's ability to transform and increase power levels from Dragon Ball Super. This would make his battles even more epic.
One interesting story could be the ones where the characters from Dragon Ball Z end up in the High School DxD universe and have to adapt to its rules. For example, Goku trying to understand the whole devil - angel - human dynamic in the High School DxD world would be quite fascinating.
There could be an amazing fanfiction where Naruto somehow gets involved with the world of High School DxD and the concept of a Dragon God. Maybe Naruto's powers would be a great addition to the battles and power dynamics in the DxD universe. He could form unique alliances or rivalries with the characters from DxD.
One possible plot could be that the characters from Dragon Ball Z end up in the High School DxD world and have to deal with the different power dynamics and social hierarchies there. For example, Goku could be intrigued by the way the devils and angels in High School DxD have their own unique powers and politics. He might try to learn from them while also showing off his Saiyan strength.
Goku is often a popular character. He could be the star athlete at the high school, always using his super strength and speed in sports events. His innocent and kind - hearted nature would also make him well - liked among the students.
Gohan's super strength would make him a force to be reckoned with. He could easily overpower most of the regular characters in High School DxD. His ki blasts could also be a new form of attack that the others in the universe have never seen before.