Papyrus would be a great addition. His relationship with Sans could take on a whole new dynamic when Sans becomes human. Papyrus might be overprotective at first, not sure how to deal with his now - human brother. He could also be a source of comic relief as he tries to understand human behaviors.
Another possible main character could be Frisk. Since Frisk has a connection with Sans, they would be involved in Sans' story when she becomes a girl. Frisk could be the one who first discovers Sans' transformation and helps her come to terms with it, or they could be on an adventure together with Sans in her new form.
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.
Perhaps there is a scientific experiment gone wrong that turns Sans into a human. He then has to hide his identity from the other monsters while he tries to find a way to reverse the process. In the meantime, he experiences human sensations like hunger and fatigue for the first time, which makes for some comical and also poignant moments in the fanfiction.
Yes, in some fanfictions, Sans is often given human characteristics or re - imagined as a human. This allows for different interpretations and storylines that deviate from the original non - human character in the game. It gives creators a chance to explore new relationships and scenarios with a more human - like Sans.
Frisk could be one of the characters watching. Given their role in the game, they would be interested in Sans' fight as it could impact their journey through the Underground. Additionally, Undyne might be present. She's a strong fighter herself and would probably be sizing up Sans' techniques, maybe even learning from him or being critical of his fighting style depending on her mood.
It could be another monster who is jealous of Sans' new form. This monster might feel that Sans is getting special treatment or opportunities because he's now a human. So, this jealous monster tries to find a way to either turn Sans back into a skeleton against his will or make his human life miserable by causing trouble for him in the human world, like spreading false rumors about him.
One idea could be a crossover where Sans from the game enters a human world during a major historical event like the Industrial Revolution. He could interact with inventors and workers, his magic and unique personality causing all sorts of comical and thought - provoking situations.
Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.
I'm not sure of specific 'lust sans x fell sans fanfiction' titles. But you can try searching on fanfiction websites like Archive of Our Own. They usually have a wide range of fanfictions and you might be able to find some related to these characters there.
There's a really cool fanfic called 'FellSwap Encounters'. It delves deep into the relationship between underfell sans and underswap sans. It starts with a chance meeting and then shows how they gradually understand and support each other. The author does a great job of showing their different characteristics and how they complement one another.