Some characters might see fanfiction as a source of inspiration. For instance, Kiba could get new ideas for his swordsmanship or his way of dealing with opponents from the creative battles described in fanfiction. He might start to think about different strategies that the fanfiction writers come up with.
The characters might also be concerned about how the fanfiction affects their public image within their world. If there are fanfictions that spread false or negative rumors about them, they might have to take steps to correct those impressions. However, if the fanfiction is positive, it could enhance their reputation and make them more popular among the other students or devils. For example, Xenovia might find that positive fanfiction about her makes her more respected among the exorcist community.
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.
In a Highschool DxD fanfiction, Naruto might appear if the author wants to create an interesting mash - up. It could be that Naruto is on a new mission and somehow ends up in the Highschool DxD world. For example, he could be chasing after a rogue ninja who has found a way to hide in that different realm. Or perhaps, the fanfiction writer just wants to see how Naruto's unique abilities, like his shadow clones or Rasengan, would interact with the magic and powers in the Highschool DxD setting.
One way could be that Naruto's skills and values from his own world clash and then blend with the supernatural elements in Highschool DxD. For example, his strong sense of justice might lead him to oppose some of the more chaotic or immoral actions in the DxD world. And his ninja techniques could be a new and interesting addition, like using shadow clones in battles against devils or fallen angels. Kurama could also play a role, perhaps his chakra influencing the power dynamics in the DxD setting.
It allows for character development. A character with Mystic Eyes in a fanfiction might have to struggle with controlling or understanding their power. This internal conflict can make the character more relatable and complex, like how in the original Highschool DxD, characters grow through their various challenges.
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.
Well, it could start with a portal opening up between the two universes. Yasaka could be the first to sense it and investigate. She then meets Naruto and they start to communicate through some kind of magic or chakra - based translation. As for the Highschool DxD characters, they could follow Yasaka's lead. They could then all work together to close the portal which is causing some sort of instability in both worlds.
One way Spiderman could fit in is if he was brought there by some sort of magical or scientific accident. He could then use his spider - sense to detect the hidden motives and powers of the characters in Highschool DxD. He might get involved in the power struggles between the devils, angels, and fallen angels, trying to maintain a balance. His suit could also be modified with some magic or technology from the Highschool DxD world to give him new abilities.
Well, imagine if the Mind Stone was integrated into some sort of magical artifact in the Highschool DxD world. The characters might have to deal with the mind - controlling aspects of it. Issei could be affected by it, and his friends would have to find a way to break the hold while also dealing with the usual devils, angels, and fallen angels politics.
The story could fit by having the Dragonborn be a transfer student. They come with their unique skills and knowledge from Skyrim. Their combat abilities would make them stand out in the school's battle - related activities.
Issei would probably be the first to want to fight Thanos. He's always eager to prove himself and protect his friends. He might initially underestimate Thanos due to his own growing power and confidence, but soon realize how powerful Thanos really is. Rias would be more cautious. She would likely try to gather information about Thanos first and then formulate a plan to either fight or negotiate with him.