It depends on different perspectives. From a purist point of view, it's likely to be seen as inappropriate because it may distort the original intention of the source material. But for those who view fanfiction as a space for unrestricted creativity, it could be seen as an interesting exploration. However, it should be noted that if the work is shared publicly, it may receive a lot of criticism due to the sensitive nature of the topic.
This kind of plot in fanfiction is not very common or well - received in general. The relationship between Issei and Koneko in the original work usually has its own established dynamics. When fanfiction takes such a radical turn as getting Koneko pregnant by Issei, it can shock and even offend many fans who are attached to the original characters and their relationships. Fanfiction is supposed to be a form of creative extension, but this might be pushing the boundaries too far for most.
There could be consequences within the story itself as well. It might change the entire future arc of both Issei and Koneko in ways that are difficult to reconcile with the original character traits. For example, their relationship dynamics would be completely altered, and they would have to deal with the responsibilities of pregnancy, which could be a very different direction from what the original work intended. Additionally, other characters in the fanfiction might also be affected by this new development and their reactions could be hard to predict and write in a believable way.
There might not be any 'appropriate' fanfiction of this nature. Naruto and Koneko are from distinct anime worlds, and creating a plot where Naruto gets Koneko pregnant is not only a strange combination but also might violate the integrity of the original characters and their respective storylines. In the world of fanfiction, there are many other positive and creative ideas to explore rather than this rather inappropriate one.
I haven't actually read a specific fanfiction with this plot. However, if such a fanfiction exists, it would likely involve a lot of creativity in terms of how the relationship between Issei and Serafall developed to that point. It could explore different aspects of their personalities and how they would handle the situation of a pregnancy, which might be quite different from the original storylines of their respective characters.
The main problem is that it completely distorts the original characters. Naruto has his own set of relationships, goals, and a world that revolves around ninja and his friends in Konoha. Koneko has her own story in the High School DxD world with different rules about magic and the supernatural. Combining them in this way for this kind of plot is disrespectful to the original creators' intentions and the essence of the characters.
One common theme is the growth of their relationship. It often starts from a somewhat distant or one - sided view and develops into a deep and meaningful connection. Another theme is their combined strength in battles. Writers like to show how they complement each other in fighting against enemies.
One great Issei and Koneko fanfiction is 'The Bond of Opposites'. It delves into their relationship in a really interesting way. It shows how Issei's brash nature and Koneko's quiet demeanor play off each other.
In a fanfiction, it could be that Koneko and Issei are chosen for a special mission by the Gremory group. They are initially hesitant to work together as they have different approaches. However, as the mission progresses, they start to complement each other's styles. Koneko's stealth and Issei's more upfront fighting style work in harmony to defeat the enemy and complete the mission, and in the process, their relationship grows.
In a fanfiction, Issei and Koneko could be paired for a school project. At first, they struggle to work together because of their different personalities. Issei is more outgoing and impulsive, while Koneko is quiet and reserved. But as they dig deeper into the project, they start to understand each other better and develop feelings for one another.
It depends on the norms and values of the specific fandom. In some fandoms, such a plot might be considered too extreme or against the spirit of the original work. However, in more open - minded fandoms, as long as it is well - written and doesn't violate basic ethical boundaries, it could be tolerated. But it's important to note that any fanfiction should respect the overall essence and the majority of the fans' feelings of the original creation.
I'm not sure if there is a specific fanfic with that exact plot. There are a vast number of fanfictions out there, and it would take some extensive searching through relevant fanfic platforms to determine if one exists.