Another theme could be the idea of loyalty and friendship being tested. Naruto is used to having strong bonds with his friends in his own world. But in High School DxD, the rules of friendship might be different. His new acquaintances might betray him for their own self - interests, which would shatter Naruto's trust and force him to re - evaluate his relationships in this new and strange world.
Power struggle is also a frequent theme. Naruto is a powerful ninja, and in the High School DxD world, power is highly coveted. His powers might attract the wrong kind of attention, and those who want to gain or maintain power could betray him to eliminate a potential rival.
Friendship is also a big theme. Characters from both universes form new friendships and alliances. For example, Naruto's idea of comradeship might influence the characters in High School DxD and vice versa. They learn from each other's values and work together to face challenges, whether it's school - related in the High School DxD setting or something more global like in Naruto's world.
One common theme is power growth. In both Naruto and DxD, the characters are constantly striving to become stronger. Fanfictions often combine these two elements, showing how characters from one series adapt to the power - up systems of the other. For example, Naruto characters might learn about the Sacred Gears in DxD.
One common theme is power. Since both Naruto and High School DxD have complex power systems, fanfictions often explore how the characters' powers interact. Another is friendship, as in how Naruto's idea of friendship might influence the relationships in High School DxD. And of course, there's the theme of adventure, with Naruto getting involved in the wild adventures that usually happen in High School DxD.
One possible fanfiction idea could be that Naruto, who somehow ends up in the world of High School DxD, is betrayed by some of the devils there. Maybe they initially seem friendly but then backstab him for his power or because they fear his potential. Naruto, being the resilient character he is, would then have to find a way to overcome this betrayal and prove himself.
Naruto's reaction could be a mix of sadness and resolve. He has a big heart, so the betrayal would cut deep. But he also has the will of fire within him. He could use this as fuel to rise above the situation. He might seek out new allies in the High School DxD world, like Issei and his friends (if they weren't part of the betrayal). Together, they could work towards uncovering the truth behind the betrayal and taking on any threats that might be related to it.
One common theme is power. Both Naruto and High School DxD have strong power systems. In fanfictions, it's often about how Naruto's power from the ninja world compares or combines with the power of the devils, angels, and other beings in the DxD world. Another theme is friendship. Naruto is all about friendship, and in these crossovers, he forms new friendships in the DxD setting, like with Issei and his friends.
Transformation is also a popular theme. Given ACNologia's nature, fanfictions may involve him undergoing some sort of transformation, either physically or in his personality. This can be due to the influence of the High School DxD environment or his interactions with the characters. And this transformation can then drive the plot forward in interesting ways.
One common theme is the exploration of relationships. Given the nature of the original series, many fanfictions focus on the romantic or friendship relationships between Uri and other characters.
One common theme is power growth. Many fanfictions focus on the main character getting stronger in the High School DxD universe, whether it's through new powers or better control of existing ones.
One common theme is Asia's growth. Since she starts as a rather timid character in the original, fanfictions often focus on her becoming stronger. Another theme is her relationships, like her friendship with the main characters.