okay so the old “Sans used to sing Papyrus to sleep and Paps claims he remembers him having a really good voice but hasn’t heard his brother sing in a very long time” thing.
Then reader finds out and asks to hear him, says he must be good to have left an impression on his brother. He refuses, and every so often she’ll bring it up and entice him because she loves music and hearing her best bud sing? amazing. perfect. can’t wait for the opportunity.
yadda yadda he catches feels, Undyne and Alphys playing dating coach, yadda yadda, convinces him to confess feelings via a ~serenade~. only of course it has to be public so they can’t coo and awww over the entire thing, so off to a bar for karaoke night.
sans picks out the sappiest love song he can find that he knows reader likes, basically stares at her the entire time while on stage. starts out basically as mumbling but as he sees her positive reactions, gets more bold.
then
“oh shit, Sans. You really do have a great voice! I have no idea why you were so shy about it. I’m so happy I finally got to hear it.”
...
he sweats. Did... reader not get it? I mean, he was very obvious. A cheesy love song, blushing and sweating bullets and staring into her eyes the entire time. Did he just... humiliate himself on stage for nothing? Damn.
and??? idk how it would end but options:
1. she remains oblivious, makes a generic “you’re a great friend, Sans” comment at some point. tragic.
2. she seems oblivious, but goes up for her turn and has picked a duet. song starts and she drags Sans back on stage and now they’re serenading each other. awww.
3. reader seems oblivious and very sweet/naive. goes on stage for her turn... and a raunchy song comes on. Animal by NIN or I Touch Myself by the Divinyls. commence lots of intense eye contact and air humping and pointing. Sans dies a little on the inside.