Well, the characters in Genshin Impact might be quite surprised at first. They come from different universes. For example, the art styles are different. Genshin Impact has a more fantastical, open - world kind of style, while Honkai Impact has its own unique and often more action - packed feel. If they were to react to the fanfiction, some might be amused, like Paimon. She would probably find the crossovers in the fanfiction really interesting and keep asking questions about the Honkai Impact characters. And Diluc might be a bit more reserved, but still intrigued to see how the two universes are combined in the fanfiction.
Some characters might be very intrigued. For example, Albedo, with his scientific mind, would likely analyze the concepts in the fanfiction related to Earth. He'd be interested in how Earth is depicted, its cultures, and technologies. He might even try to find similarities with Teyvat in terms of elemental powers or social structures.
One interesting Neuvillette fanfic could be 'Neuvillette's Journey of Discovery'. It might explore his character in-depth, his thoughts and how he copes with the various situations in the world he inhabits. Another could be 'Neuvillette and the Mysterious Case' which could involve him solving some sort of enigma in Fontaine.
Shigaraki would likely be dismissive. He's so focused on his own goals of destroying the hero society that he'd see fanfiction as a useless distraction. However, if there were elements in the Genshin Impact fanfic that could somehow be related to his own plans, like powerful artifacts or magic that could be used for destruction, he might take a second look.
Some characters would be fascinated. For example, Fischl would likely see it as a grand adventure from another realm. She might even incorporate some of the elements from the Honkai Impact fanfiction into her own delusions. Keqing, on the other hand, would probably approach it more analytically. She would try to understand the logic behind the Honkai Impact world and how it differs from Genshin Impact. She might make notes about the different power - leveling systems and how the characters interact in the fanfiction.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfics from various fandoms, including those about Neuvillette. Many talented writers post their works there.
The students of Class 1 - A would have a range of reactions. Ochaco might find the romantic elements in the fanfiction really sweet, especially if there are any cute ship moments. Iida would likely be concerned about whether the fanfiction adheres to some sort of moral code. He might be nitpicky about things like fairness and justice within the fictional story.