Perhaps the reader has some special ability that Sans notices. For example, the ability to see things that others can't. Sans then teams up with the reader to figure out what these strange visions mean and in the process, they grow closer. They might face some challenges from other characters who are suspicious of the reader's power, but Sans is always there to protect and support.
One idea could be that the reader is a new arrival in the Underground and Sans is immediately intrigued by their unique personality. He starts to show them around in his own laid - back way, sharing jokes and secrets of the Underground along the way.
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.
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.
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.
One idea could be that the reader is a scientist who discovers a way to communicate with gremlins. The gremlins are initially suspicious but as the reader shows no harm, they start to cooperate. Together, they try to solve a mystery in the gremlin's underground lair that is affecting both their worlds.
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 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.
The reader could be a lost traveler who stumbles upon Dracula's castle during a storm. Dracula offers shelter, but the reader soon realizes that there's something not quite right. As time passes, the reader discovers Dracula's loneliness and begins to feel sympathy for him. This could lead to a friendship or more, as the reader tries to bring some light into Dracula's dark existence.
One plot idea could be a redemption arc for Underfell Sans. Frisk could be the catalyst for his change, helping him to see a different way of living in the Underground. For example, Frisk might show him acts of kindness that he's not used to, and this could start to break down his tough exterior.
One unique idea could be a time - traveling sans x reader fanfiction. The reader could accidentally travel back in time to when Sans was first created or to a crucial moment in his past, and they have to navigate through that while also developing their relationship.
One interesting plot could be the reader helping Gaster with his scientific experiments in the Underground. It could show their growing bond as they work together and face various challenges in the lab.