The storylines can be combined by creating a common threat. Suppose an enemy emerges that is interested in both the power of the Bijuu and the power sources in Highschool DxD. This would force the characters from both universes to team up. Also, the cultural differences between the Naruto world and the Highschool DxD world can be explored. Naruto could introduce concepts like ninja clans and shinobi training to the DxD characters, while they could introduce him to the complex social hierarchies and supernatural politics in their world.
Naruto would probably be very friendly and curious. He'd be excited to meet all the different types of beings in the Highschool DxD world. He might start by trying to make friends with Issei, sharing his stories about his own battles and adventures in his world.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
One similarity could be the themes of growth. In Naruto fanfiction, just like in the original Naruto series, the characters often grow in power and maturity. In Highschool DxD, the main character also experiences significant growth in his abilities and relationships. Another similarity might be the existence of different factions or groups. In Naruto, there are the different ninja villages, and in Highschool DxD, there are different supernatural groups.
If Naruto entered Highschool DxD with mokuton powers, it would be a wild ride. His powers would make him a target for some who want to use his abilities for their own gain. But Naruto being Naruto, he would make friends with the good - hearted characters. He could teach them about his ninja ways and learn about their magic and devil powers in return. His mokuton would also allow him to create unique training areas for himself and his new friends to get stronger.
The settings are also very different. Naruto is a more traditional Japanese - inspired ninja world, while Highschool DxD is set in a modern - day high school with a lot of supernatural elements. Writers need to blend these settings well so that the story doesn't feel disjointed. For example, if a character from Naruto enters the Highschool DxD world, how do they react to modern technology and the different social norms?
Another good one could be 'Highschool DxD: The Naruto Influence'. In this fanfiction, the author manages to blend the storylines in a way that makes it seem like Naruto has always been a part of the Highschool DxD world. His presence changes the relationships between the characters in a fascinating way.
Well, you can expect some really interesting character crossovers. Naruto's unique ninja abilities and his never - give - up attitude will surely make an impact. In a harem situation, he'll probably be surrounded by characters from Highschool DxD who are intrigued by his different ways. There could be a lot of cultural clashes too, since the Naruto world and Highschool DxD world have different norms.
One characteristic could be that the story remains true to the Naruto universe's tone and themes. It would likely have Naruto dealing with his own set of challenges like fighting against the Akatsuki or protecting his village. It wouldn't introduce the harem - like or devil - based elements from 'Highschool DxD'. Also, the characters would be from the Naruto world, with their own personalities and abilities intact, without any 'Highschool DxD' - influenced changes.