Well, one characteristic is the blend of different universes. Since sans and reigen are from different fictional worlds, fanfics often play with the idea of how these two very different worlds collide. For example, in some, the magic or powers from each world interact in unique ways.
One popular sans x reigen fanfic is 'Crossed Realms'. It weaves an interesting story where the two characters from different universes interact in unexpected ways. It has great character development and a unique plotline.
A common theme is friendship. Their relationship is often explored in depth, showing how they rely on each other. For example, in many fanfics, mob looks up to reigen for guidance.
One characteristic could be the unique blend of the horror and lust elements from the two 'sans' characters. It might play with the contrast between the spooky or menacing nature of horror sans and the more sensuous or alluring aspects of lust sans.
A popular Ink Sans x Error Sans fanfic is 'Shades of Their World'. In this fanfic, the author does an excellent job of exploring the different aspects of their personalities. Ink's chaotic but positive energy is contrasted with Error's more brooding and negative nature. It makes for a really engaging read as you watch their relationship develop. There's also 'Ink's Influence on Error'. This one shows how Ink's presence starts to change Error in unexpected ways. It's a short but sweet fanfic that leaves a lasting impression.
One popular sans x sans fanfic could be those that explore their unique friendship in different AUs (Alternate Universes). For example, some might focus on how they interact during a time of crisis in the Underground. Another type could be the ones that delve into their shared experiences and how they support each other through hardships. There are also fanfics that might take a more comical approach, showing their humorous banter and silly antics together.
There's also 'The Unspoken Connection' which is really good. It focuses on the unspoken understanding between mob and reigen. The author does a wonderful job of portraying their unique dynamic.
A 'flowey x sans fanfic' is often a creative work within the Undertale fandom. It might explore unique relationships between Flowey and Sans. These fanfics can range from friendship - based stories to more complex and imaginative scenarios. The authors usually take the distinct personalities of Flowey, the somewhat menacing flower, and Sans, the laid - back skeleton, and blend them in unexpected ways.
To write a great Error Sans x UnderSwap Sans fanfic, start with a strong concept. Maybe it's a story about how UnderSwap Sans tries to 'fix' Error Sans in some way. Research the existing fan works and the official Undertale lore to get inspiration. When it comes to the writing itself, build up the relationship between the two characters gradually. Show how they initially react to each other, and then how their relationship evolves over time. Use vivid descriptions for their actions and the settings around them. For example, describe how Error Sans' glitches affect the environment they are in, and how UnderSwap Sans' positive energy can sometimes counteract that. Also, make sure the pacing of the story is right, not too slow or too fast.
I'm not aware of any specific 'horror sans x lust sans fanfic' to recommend. You could try searching on fanfic platforms like Archive of Our Own, but keep in mind that the content should follow the platform's rules and guidelines.