One common theme is the struggle between good and evil, as Dream Sans is often associated with positive aspects and Nightmare Sans with negative ones. Their relationship in fanfics can explore how they deal with their opposing natures.
Some Tumblr blogs are dedicated to sharing Undertale fanfics as well. You can search for tags like '#dream sans x nightmare sans fanfic' on Tumblr. These blogs often have a community of fans who are into the same kind of content. They might also provide reviews or recommendations for other great fanfics in this category.
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 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.
In 'cross sans x cross chara fanfic', a common theme is the exploration of the characters' backstories. Since both cross sans and cross chara have their own histories in the Undertale lore, fanfic writers might delve into these backstories to create more depth in their relationship. Another theme can be redemption. If either character has made mistakes in their past, the fic could focus on their journey towards redemption. And of course, there is always the theme of the relationship between the two characters, whether it's a platonic bond or something more complex that develops over the course of the story.
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.
Common themes in 'classic sans x fell sans fanfiction' include adaptation. Since they are from different universes, they have to adapt to each other's presence. Also, trust is a big theme. At first, they may not trust each other at all, but as the story unfolds, they build trust. Additionally, the theme of identity comes up. They have to reevaluate who they are in relation to the other character.
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.
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.