Well, it could be about Gaster suddenly reappearing in the lives of Sans and Papyrus. Maybe he has some important knowledge or a mission for them. For example, he might need their help to fix something in the Underground that's going wrong.
Obviously, Sans and Papyrus are the two main characters who we already know well from the original Undertale. They have distinct personalities that will play a big role in how they interact with Gaster. Gaster, on the other hand, is the enigma. His presence will likely drive the story as Sans and Papyrus try to figure out who he is now and what he wants, so he's also a main character in this fanfiction.
One great idea for a fanfiction could be a story where Underswap Sans and Underswap Papyrus go on an adventure in the Underground. Maybe they encounter a strange new monster that no one has ever seen before. They have to use their unique abilities to figure out how to deal with this new threat.
Papyrus might be initially shocked. He's used to Sans being more laid - back, so seeing him in a fight could be quite a surprise. His eyes would widen and he might gasp.
One common theme is mystery. Since both Gaster and Sans have mysterious backstories in the original game, fanfictions often explore and expand on these mysteries, especially in relation to their relationship.
Well, in 'underfell sans x papyrus fanfiction', their relationship is a key aspect. It can vary greatly depending on the author's interpretation. Some may focus on the loyalty between them, despite the cruel environment of Underfell. Others might explore how their personalities clash or complement each other. For example, Sans' cynicism in Underfell might contrast with Papyrus' more naive side, creating interesting storylines and character developments.
A great idea could be a story about their daily adventures in the Underground. For example, they could be on a quest to find a rare ingredient for a new recipe Papyrus wants to try. This would focus on their brotherly bond and Papyrus's enthusiasm for cooking, while Sans tags along with his laid - back attitude.
At first, Sans would probably be a bit shocked, but then he'd turn on his laid - back, sarcastic attitude. He might make jokes at Gaster's expense, which could either annoy Gaster or make him respect Sans more. If Gaster tries to harm him, Sans could use his shortcuts to dodge attacks and try to find a way to reverse the situation. He'd also likely be thinking about how this affects his friends in the Underground, especially Papyrus, and try to find a way back to them.
One possible reason could be for some sort of experiment. Gaster is often depicted as a mysterious scientist in the Undertale lore, and Sans has unique abilities. Maybe Gaster wants to study Sans' power over time and space. Another reason could be related to secrets. Sans might know something about Gaster's past or the true nature of the Underground that Gaster wants to keep hidden, so he kidnaps Sans to prevent him from revealing it.
A sense of otherworldliness is common too. Since Gaster Sans is from a rather strange and unique universe, the fanfictions often play with the idea of the reader being in a very different, almost alien world. This can lead to themes of adjustment, as the reader tries to fit into this new world while being in a relationship with Gaster Sans.