There are Sick Sans fanfictions that focus on his relationships. He could form an unlikely alliance with another character, like a character from a different AU. They might start off as enemies but gradually come to understand and respect each other through a series of events.
Some Sick Sans fanfictions explore his backstory. Maybe he has a hidden past that explains his current state. It could be that he was experimented on in some dark and secret facility, which led to his'sick' appearance and abilities. This kind of backstory adds depth to his character and makes the story more engaging.
One interesting Sick Sans fanfiction could be about his unique battles. In some stories, he uses his special powers in very creative ways against his foes. For example, he might manipulate the environment in ways that are both unexpected and fascinating.
One plotline could be how the other characters in the story react to Sans being sick. For example, Papyrus might be overly worried and try all sorts of crazy remedies. Another could be Sans trying to hide his sickness because he doesn't want to be a burden.
One interesting Dark Sans fanfiction could be 'The Darker Path of Dark Sans'. It explores his character in a more in - depth way, showing his inner turmoil and how he copes with the strange world he exists in. Another is 'Dark Sans: The Unseen Powers' which delves into his powers that are not fully shown in the original concept.
There's also a fanfiction titled 'Ink Sans and the Lost Colors'. In this story, Ink Sans loses his ability to create colors for his paintings, which is a crucial part of his power. He then embarks on a quest to find the source of his color - losing, which leads him to encounter many challenges and make new friends along the way.
One interesting 'sans fight fanfiction' story could be about an alternate universe where Sans has different powers during the fight. For example, he might be able to manipulate time in a more extreme way than in the original game.
One interesting little sans fanfiction could be about Sans having an adventure in a new, mysterious underground world. He might encounter strange creatures and have to use his unique abilities to get through various challenges.
I'm not really into that kind of fanfiction, but you can try looking on fanfiction - specific websites like Archive of Our Own. There might be some unique takes on the idea there.
There are many frisk and sans stories in fanfiction. Some might focus on their friendship adventures in the Underground. For example, there could be a story where they team up to solve a mystery that's been plaguing the monster community. They use their unique skills, with Frisk's determination and Sans' laid - back but clever thinking.
One interesting sans crossover fanfiction could be where Sans from Undertale crosses over with the world of Harry Potter. He might use his unique abilities like teleporting to surprise the wizards in Hogwarts. Another could be a crossover with the Pokémon world, where Sans teams up with Pikachu. Maybe he'd use his bone attacks along with Pikachu's electric shocks to battle villains.
Alphys could be another common character. As the royal scientist, she might be called upon to figure out what's wrong with Sans. She could bring her scientific knowledge to the table, trying to diagnose and find a cure. Her own insecurities might play a role here too, as she might be worried about failing to help Sans.
There might be a fanfiction where Time Kid Sans has to face his own past versions. It could explore how he deals with the differences in personalities among his past selves and the challenges that come with it. This could involve a lot of self - discovery and growth for the character.