Well, in a 'underfell sans meets underswap fanfiction', it might involve a clash of cultures, so to speak. Underfell Sans is from a darker, more dangerous world where survival is tough. When he meets the Underswap characters, who are often more laid - back and kind - hearted, it could lead to some really cool storylines. For example, he could learn about friendship in a new way from the Underswap Sans, or get into trouble while trying to adapt to their more peaceful world.
One possible plot twist could be that Underfell Sans turns out to be a long - lost relative of one of the Underswap characters. Another could be that a villain from Underfell follows him to the Underswap world and causes chaos. Or perhaps Underfell Sans is actually there to steal something important from the Underswap world but has a change of heart halfway through.
Another theme is friendship or partnership. They often come together to face challenges in the Underground. For example, in many fanfictions, they join forces to defeat a powerful monster or solve a mystery that threatens their world. This theme of unity despite their differences is really appealing.
The contrast between the two characters. Underswap Sans is more laid - back and friendly compared to Underfell Sans who is more rough and edgy. When they interact in fanfictions, it creates a lot of interesting dynamics, like how they might clash or gradually understand each other.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions for various fandoms, including the Undertale AU crossovers like Underswap Sans x Underfell Sans. Another place could be Fanfiction.net, though the quality may vary more there. Additionally, Tumblr sometimes has users sharing or recommending such fanfictions in the Undertale fandom tags.
Characterization is key. A good fanfic should accurately portray the personalities of both Underswap Sans and Underfell Sans. Their unique traits, like Underswap Sans' more positive and friendly nature compared to Underfell Sans' tougher exterior, should be well - presented.
There's a really cool fanfic called 'FellSwap Encounters'. It delves deep into the relationship between underfell sans and underswap sans. It starts with a chance meeting and then shows how they gradually understand and support each other. The author does a great job of showing their different characteristics and how they complement one another.
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.
Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.