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.
You can start by looking on fanfic platforms like Archive of Our Own (AO3). It has a large collection of Undertale - related fanfics, including those about Underswap Sans and Underfell Sans. Just use the search function and type in the relevant tags.
One popular 'underfell sans x underswap sans fanfic' is 'Twisted Bonds'. It explores their relationship in a really interesting way, with great character development.
Well, in Ink Sans x Error Sans fanfiction, the characterizations are key. Ink Sans' love for creativity and art can be a driving force in the story. Error Sans' glitches and his view on the 'imperfections' in the multiverse can create conflict. Writers often explore how these two elements collide. For example, Ink Sans might try to show Error Sans the beauty in chaos and creativity, while Error Sans could be constantly trying to correct or destroy what Ink Sans creates.
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.
One popular theme in Ink Sans x Error Sans fanfiction is the idea of balance. The two characters seem to be polar opposites, and fanfiction writers explore how they could potentially bring balance to each other or to the multiverse as a whole. Also, the theme of acceptance is quite prevalent. Ink Sans accepting Error Sans' destructive nature and Error Sans accepting Ink Sans' creative chaos can lead to some really heartwarming or thought - provoking storylines.
One main theme is identity. Since it's an underswap, their identities are different from the original, and exploring how they adapt to these new roles is a big part. For example, if Papyrus is more like the original Sans in some aspects, how does he deal with that change.
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.