Well, when it comes to a 'Mafiatale Sans x Reader Fanfic', you need to focus on the setting. Mafiatale has its own rules and a very different atmosphere compared to the normal Undertale universe. Sans in this AU is probably involved in illegal activities. For the reader - insert, make the character relatable. They could be an ordinary person who gets caught up in Sans' world. You can also play with the idea of the reader having some special ability or knowledge that Sans finds useful. And don't forget about the dialogue. Make it snappy and in - character for Sans. Use his unique speech patterns to bring the story to life.
First, understand the characters. Sans in Mafiatale has a unique personality. He's likely to be more menacing than the regular Sans. For the 'x reader' part, think about the relationship. Is it a love - hate relationship? Start with a strong opening scene that sets the mood of the Mafiatale world. Maybe describe the dark alleyways or the seedy underworld where the story takes place. Then, introduce the reader's character in an interesting way, like they're new to this dangerous world. Develop their interactions with Sans gradually, and don't rush the relationship. Build up the tension and the mystery as the story progresses.