It adds variety. The undertale fandom is huge and diverse, and frans lemon fanfiction offers a different take on the characters and their relationships. It gives fans who are interested in this particular pairing more content to enjoy.
Some characters might be amused. For example, Sans would probably make some witty remarks about the fanfiction's plot twists. Papyrus, on the other hand, might be a bit confused at first but then get really excited if he finds a heroic role for himself in the fanfiction.
It gives fans a chance to explore different 'what - if' scenarios. By seeing the Avengers' reactions, fans can get a deeper understanding of the characters.
The Naruto fandom would probably react very negatively. Fans are used to seeing Naruto as a hero, a symbol of hope and growth. A fanfic that portrays him as a slave goes against the very essence of what Naruto represents. There would be a lot of criticism on social media platforms, and many fans might even call for the fanfic to be removed or boycotted.
There would be a portion of fans who are very against it. They are so used to the idea of Sakura and Sasuke being a potential couple that any deviation from that would seem wrong to them. They might feel that it goes against the 'canon' in a way, even though it's a fanfic.
The Naruto fandom would likely be very divided. Some fans might be intrigued by this unique and unexpected plot twist, seeing it as an interesting exploration of an alternate Naruto universe. However, many fans would probably be quite shocked and even appalled, as it goes against the traditional understanding of the characters and the series.
Some fans might be really sad about it. They love the idea of Sasuke and Sakura together as a couple and see their relationship as a symbol of hope and growth. So, divorce fanfiction goes against their ideal view of the couple.
Well, if Undertale could watch Undertale fanfiction, it could lead to some chaos. For example, Sans might make jokes about the wild scenarios in the fanfics. Papyrus, on the other hand, might be eager to correct any misinformation about himself. And Flowey could use it as a way to manipulate things further, seeing how the fans view the characters and using that knowledge to his advantage in this strange, meta - situation.