One common theme is the exploration of their relationship in the Underlust AU context. Since it's an AU with a focus on different interpretations of the characters' personalities and relationships, fanfictions often delve into how Papyrus and Sans interact with each other romantically or platonically in this altered world.
You can try looking on Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, including Undertale and its AUs like Underlust. Just use the search function and type in 'Underlust Papyrus x Sans' to see what's available.
Often, the cuteness of the baby form is a key feature. Despite being from the Underfell universe which is usually more edgy, the baby version brings out a different side.
One aspect to consider in 'classic sans x fell sans fanfiction' could be the character development. Classic Sans is often depicted as more laid - back while Fell Sans has a harsher edge. Their interaction in the fanfiction might lead to interesting changes in their personalities. For example, Fell Sans could learn to be more relaxed through his encounters with Classic Sans, or Classic Sans could become more assertive in the face of Fell Sans's aggression. Another point is the plot. It could revolve around a common threat that forces them to work together, despite their differences. This could create a lot of tension and drama as they have to overcome their initial distrust of each other.
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.
One popular 'baby underfell sans fanfiction' is 'The Tiny Terror'. It has an interesting take on the character in a baby form, with unique plot twists and character interactions.
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.
Writers often portray their relationship as a contrast. Unfresh sans is usually more serious and reserved, while fresh sans is carefree and energetic. They might start as two characters who don't understand each other but gradually develop respect.