It could be that in the fanfiction, Naruto is under some sort of mind control when he kills Issei. This would add an interesting layer to the story as Naruto would have to deal with the consequences of his actions once he is free from the control.
Well, in such a fanfiction, the plot could be set up in many ways. Perhaps Issei's actions in the crossover world are misinterpreted by Naruto as a threat. Naruto, being a protector of his village and friends, would not hesitate to eliminate a perceived threat. On the other hand, it could also be a dark - themed fanfiction where Naruto is somehow corrupted or forced into killing Issei against his normal moral code.
No, it doesn't seem to be a common concept. Usually, Naruto fanfiction focuses on various aspects like character development, new relationships, or different story arcs that are more in line with the general themes of the original series, which is not mainly about 'kills'.
It could be a variety of villains. Maybe some of the traditional Naruto villains like Orochimaru or Pain could be in the story and Naruto kills them. Or perhaps in this fanfiction crossover, there are new villains created that are a combination of Naruto - style threats and Justice League - type menaces. It's hard to say exactly without reading the fanfiction, but it would be interesting if it included some well - known Naruto baddies being taken out by Naruto in a new setting with the Justice League around.
In some fanfictions, it's possible for Naruto to be Issei's father. Fanfiction often takes creative liberties and can create some really out - there family relationships. However, in the original Naruto and High School DxD (where Issei is from) canons, there's no such connection.
In such a fanfiction, it could be that Issei is bashed because he represents a different kind of hero compared to what Naruto stands for. Naruto is all about self - sacrifice and believing in others no matter what, while Issei is more about his own desires and power - seeking in a way. So when these two worlds collide in the fanfic, Issei might be made to look bad in comparison. Maybe the story has Naruto characters like Sasuke or Kakashi giving Issei a hard time because they don't approve of his behavior.
Yes, there might be such fanfictions out there. The idea of crossing over two very different anime universes like Naruto and High School DxD (where Issei is from) and creating a family - like bond between the characters is an interesting concept that fanfiction writers might explore.
In a fanfic where Naruto kills Sirzechs, it could be a very interesting concept. Maybe it's a crossover universe where the power dynamics are completely different. Naruto, with his powerful jutsu, might have a reason like protecting his own world from a perceived threat by Sirzechs. However, it also depends on how the author has set up the story, the rules of the world they've created, and the motives given to Naruto for such a drastic act.
If Naruto kills Rias in a fanfiction, it might be due to a misunderstanding. Naruto is known for his strong sense of justice. Perhaps in this fanfic, Rias is misrepresented as a threat to Naruto's world or his friends. But it's still quite a shock as they are from different fictional universes with their own moral compasses.
In fanfiction, such a plot would be extremely dark and out of character for Naruto. Naruto is typically portrayed as someone who values his friends and comrades, especially Hinata who he has deep emotional connections with. It could be a very edgy take in the fanfic world, perhaps exploring some extreme 'what - if' scenarios or alternate universes where Naruto has been corrupted or misinformed in some way.