A potential plotline could be Naruto arriving in the Highschool DxD world and being mistaken for a new type of devil or other supernatural being due to his 'dark godlike' nature. He would then have to prove his intentions and find his place in this new world. Another plotline could involve Naruto using his skills to train the characters in Highschool DxD in combat, similar to how he trained his friends in the Naruto world.
In this fanfiction, Naruto's 'dark godlike' form might change the power balance in Highschool DxD. Since Naruto has always been about growth and power - up in his original story, in this crossover, he could quickly rise to a very high - level position. His values from the Naruto world might clash with the values in Highschool DxD. For instance, his belief in friendship and hard work could be a contrast to some of the more self - serving or power - hungry characters in Highschool DxD. This could lead to some interesting story arcs.
The Highschool DxD manga has indeed finished. It wrapped up after completing all the planned story arcs and providing a satisfying conclusion for the readers.
Well, in such a fanfiction, Dark Naruto might use his powerful and often shadowy abilities to outwit Rias. Maybe he'd expose some of her less - than - noble actions in the Highschool DxD world. He could easily overpower her in battles, using his ninja skills and the dark powers he has. For example, his shadow clones could be used to confuse Rias and her peerage while he strikes from the shadows.
If Spawn were to interact with the main characters, he would bring a whole new level of seriousness. He has a dark past and a lot of baggage. When it comes to Issei's goal of becoming a harem king, Spawn might find it absurd but also interesting in a way. He could offer his own perspective on power and how to use it responsibly, which would be a contrast to the more carefree attitude of some of the Highschool DxD characters. And with the other characters like Akeno and Koneko, he'd be sizing them up, trying to figure out if they're threats or allies.
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.