Well, 'Naruto in DxD: The Neglected Savior' is quite interesting. In this fanfiction, Naruto is underestimated by the DxD characters at first. However, as the story progresses, his unique abilities from the Naruto world start to make a big impact. There's also 'DxD: Naruto, the Neglected Force' where Naruto has to deal with being ignored while secretly being a very powerful entity.
There's a fanfiction where Naruto is transported to the Highschool DxD world and is initially neglected due to his different upbringing. In this story, he gradually forms alliances with some of the outcast characters in the DxD world. They, together, start to change the power dynamics. It shows how Naruto's values from his own world can have an impact in a new and different setting like Highschool DxD.
You can try fanfiction websites like FanFiction.net or Archive of Our Own (AO3). These platforms have a large collection of fanfictions, and by using the search terms 'Naruto DxD Neglected', you should be able to find relevant stories.
A possible plot twist is that the characters from High School DxD initially underestimate Naruto due to his neglected state. But then they find out that he has a deep understanding of loyalty and friendship from his time in Konoha. This makes him form unexpected alliances in the DxD world, like teaming up with Issei to take on a common enemy that neither could face alone.
The power systems might also clash. In the Naruto world, power often comes from training and mastering one's own internal energy. In High School DxD, it's more about the power of the Devils, Angels, and Fallen Angels. So when Naruto enters the DxD world, his idea of power through hard work might be at odds with the more innate powers of the DxD characters. He could try to teach them his ways, or they could try to convince him that their powers are superior.
You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just use the search terms 'Naruto Highschool DxD Neglected' and browse through the results. There are often tags and filters on these sites that can help you narrow down your search further.
Perhaps he's been influenced by someone who gives him bad advice. For example, if someone tells him that natural talent is enough and training is overrated. Also, he could be dealing with some personal problems, like a conflict within his family or among his friends that takes up all his time and mental energy, causing him to neglect training.
He might start to rely on his friends more. For example, Sasuke and Sakura could become his main source of emotional support. He would focus on training with them and sharing his feelings, which in turn helps him to grow stronger and more independent.
In Naruto, the Jinchuuriki were often treated poorly. In fanfiction, this neglect can be explored in many ways. One common approach is to focus on the emotional and psychological impact on the Jinchuuriki. For example, they might feel isolated from the rest of the village due to the fear and prejudice associated with the tailed beasts within them. This can lead to some really interesting character development stories in fanfiction.
In a fanfiction, perhaps neglected Naruto runs away from Konoha and ends up in the same area as Yugito. Yugito, having also faced her own share of hardships, takes Naruto under her wing. They bond over their experiences, and Yugito helps Naruto to better understand his chakra nature by sharing her knowledge of the tailed beasts' chakra. This could lead to Naruto developing new abilities and a newfound confidence.