Few things before we get started big thank you this YouTuber who was one of my Resources for this theory near the end without him I wouldn’t have worked our that Dream theory is involved I was tossing up between dream theory and Coma theory but dream theory looks more of a possibility if you want to see his video link here——> https://youtu.be/PXiM4GUcilQ
By now hopefully everyone would have heard of DeltaRune a chapter based game with the same astectic and feel and look as Undertale given that DeltaRune is and anagram for Undertale but that is not the only things they have in common with each other 
Look at lancer
Now look at our beloved sans see the Resemblance you think it stops there
Let’s go with WD GASTER a weird monster that just didn’t fit in to undertale seemed so out of place
This is Rouxls Kaard a shop merchant and puzzle master yeah Coincidence maybe but I think not so what does all this mean well besides stirring up a big debate online if it’s a prequel or a sequel which by the way doesn’t matter because what you are seeing isn’t the world of undertale if you are happy with your out come in undertale it’s not going to be affected in anyway in deltarune what ever out come you picked weather it was to live in peace or erase the world it will stay as you left it.
Deltarune is a different timeline a parallel universe like legend of Zelda Majora’s mask just read this To see why we say it’s a parallel world from Toby fox twitter
I’ve scaled it down but there you go straight from the creator himself now remember I said I had a second theory