A fun fact I know is that in Deltarune if you give Noelle the Puppet Scarf(which is normally impossible because you get it after Noelle leaves your party), she says it feels like guitar strings. When Chapter 2 came out it was a random piece of flavor text, but in reality it was a connection to her missing sister, December/Dess. We found out about this when the Spamton Sweepstakes happened(a charity event where Toby Fox revealed new prizes and hints towards future chapters as more money was raised). In it we are lead to a webpage, deltarune.com/december, which is an old 404 page-not-found error message. If you change the link to deltarune.com/dess you get a red guitar on a black background. If you click on the guitar, a remix of Noelle's theme, Lost Girl, plays. People have found out that the song is called Find Her, which essentially tells us Dess did indeed go missing(it wasn't very conclusive if Dess is missing or she just left the town).
We don't know where Dess is at this point in time. The most likely place I think she is is in the unused code folder(I'm gonna have to explain that, don't I). So when Chapter 1 was released there was section of code specifically marked unused and it had a character wandering in a dark void screaming out for help. In Chapter 2 it was changed to them realizing no one can hear them but they can hear some kind of scratching. So far that's all we have but the fact that it appears to be telling a story makes people think it will actually be important later on.