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 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 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.
A plot could involve the reader being a healer of sorts. Flowey, being curious, approaches the reader. As they interact, the reader starts to show Flowey kindness that he's not used to. This kindness begins to thaw his cold, calculating nature. They could then go on adventures together in the Underground, with Flowey slowly becoming more protective of the reader. Also, the reader could introduce Flowey to new concepts like friendship and self - sacrifice, which would be a huge change for him.
One plot idea could be that the reader is a new ninja in the village and Sasuke is assigned to train them. Through the training sessions, they start to understand each other better and develop feelings. Another could be that the reader has a special power that Sasuke needs to help him in his quest for revenge, and as they work together, love blossoms.
Another plot could be that the reader is lost in a strange part of the Pokemon world and Pikachu is the one who finds them and leads them to safety. Along the way, they become great friends and share many adventures. This could involve exploring new areas, meeting different Pokemon species, and learning about the secrets of the Pokemon world.
One idea could be that the reader is a new resident in the haunted house where Beetlejuice lurks. They accidentally summon him while trying to fix up the old place. Another could be that the reader is a medium who can see Beetlejuice clearly and they form an unlikely friendship. Maybe the reader is a lost soul like Beetlejuice and they find common ground in their otherworldly existence.
One plot idea could be the reader being a new hunter who catches Hisoka's attention during the Hunter Exam. Hisoka decides to 'mentor' the reader in his own twisted way, leading to a series of dangerous and exciting adventures.
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.