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.
A plot could revolve around Damon trying to make amends for his past vampire deeds as a human. He could start by helping the victims of his previous vampire attacks or their families. Also, his love life would change. If he was in a relationship with a vampire or a human before, now as a human, new dynamics would emerge. He might worry about being too weak to protect his loved ones.
One idea could be that Godzilla, as a human, has to hide his true identity while trying to understand human emotions. He might struggle with the concept of love and friendship, which are very different from his previous existence.
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.
Another plot could be that Damon becomes human again to be with a human he loves. But once he is human, he realizes that the world he thought he knew as a vampire is very different. He has to start from scratch in building relationships, getting a job, and learning how to live without his vampire powers. He also has to face the jealousy and hatred from some vampires who see his transformation as a betrayal.
A plot could revolve around an accident that causes their universes to merge partially. Underfell Sans and Sans find themselves in a strange new reality where they have to figure out how to get back to their own universes. Along the way, they learn about the differences and similarities in their lives, and this leads to some really interesting character development.
A cool concept for 'human becomes twilight fanfiction' is a human who gets transported into the Twilight universe. This person could try to change the story's events. Maybe they befriend Bella and try to prevent some of the dangerous situations she gets into. Or they could end up falling in love with a minor character and create a whole new sub - plot. It could also be that they are turned into a hybrid of a vampire and a werewolf, which is a unique situation not really explored in the original series.
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.
A plot idea could revolve around the discovery of the pregnancy. Maybe it was unexpected, and they initially panic. But then, as they start to think about it more, they get excited. They could start making plans like decorating a nursery in their unique Underground home. There could also be a plot where they face some sort of danger during the pregnancy that they have to overcome together.
One idea could be a bitty sans getting lost in a big, strange world outside of his usual tiny environment. He has to find his way back home while facing various challenges like avoiding being stepped on by giants and finding food sources. Another plot could be about a bitty sans forming an unlikely friendship with a non - bitty character, perhaps a human who at first is scared of him but then realizes his cute and friendly nature.
Well, a plot could involve the reader being a magic user who accidentally casts a spell on Sans that makes him see the reader as his long - lost love. As they interact more, Sans starts to develop real feelings for the reader. Another option is that the reader is a singer in the Underground and Sans is drawn to their music. He starts to attend all of their shows and they gradually get to know each other. A third idea is that the reader is an artist and Sans discovers their art and is fascinated. He asks the reader to teach him about art, and this shared activity brings them closer together.