One popular one might be 'A Tale of Two Skeletons'. It delves deep into the relationship between undertale sans and underswap papyrus, exploring their different personalities and how they interact in a unique AU setting.
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.
Another option is to join Undertale - related forums or communities. People there often share and recommend good fanfictions. For example, on the Undertale subreddit, you can post a request for undertale sans x underswap papyrus fanfictions and I'm sure the community will be happy to help.
A plotline could be that they discover a hidden part of the Underground. This new area could have different rules and strange creatures. They have to adapt quickly to survive and also figure out how this new place relates to the rest of the Underground.
Common themes include loyalty. They are extremely loyal to each other in most fanfics. Another one is the theme of overcoming challenges. Whether it's dealing with other characters or facing difficulties in their world, they work together to overcome them. There's also the theme of fun and humor. Their interactions are often filled with jokes and light - hearted moments, which makes the fanfics enjoyable to read.
There's also 'Swap Brothers' Bond'. This fanfic delves deep into the brotherly bond between Sans and Papyrus in the Underswap setting. It shows their daily adventures and how they support each other. The author does a great job of capturing the essence of their swapped selves, and it's full of humorous and touching scenes.
One common theme is the exploration of differences. Underfell sans and underswap papyrus are from different universes with distinct personalities. Fanfictions often focus on how they deal with these differences, like underfell sans's more aggressive nature compared to underswap papyrus's more positive and energetic self.
There are many alternatives. You could look for fanfictions that focus on their platonic relationship, like them going on a quest together in the Underground. Another option is fanfictions that explore their individual character growth outside of a romantic or inappropriate context.
I don't recommend seeking out 'lemon' fanfiction, especially for undertale sans x papyrus. Their relationship in the original game is more of a brotherly one. If you want to explore fanfiction about them, there are plenty of non - 'lemon' ones that focus on their adventures, character development, and the unique bond they share in the Undertale universe.
A great Underswap Sans x Papyrus fanfiction might be the one that delves into their backstory. It could explore how their relationship developed from when they were younger. Maybe there was a time when they had a big fight but then learned to understand each other better. This kind of story can be really heartwarming as it shows their growth and the strength of their brotherly love.