I'm sure there's been a truly enormous amount of speculation on whether the storylines of Toby Fox's two games, Undertale and Deltarune, are meant to mesh with each other.
now, while UT is presumably complete and more or less finalized, Deltarune is incomplete, possibly very incomplete depending on how many chapters Fox is able to deliver. there's also the possibility that the fandom will influence Fox towards attempting some kind of synthesis of the two games. the prevailing fan culture loves syntheses and grand unifying narratives—I'm scarcely immune to the temptation—and commercial artists tend to feel constrained to give audiences what they expect. Deltarune and Undertale might end up closely related but in the kind of unsatisfying, retconned, trying-too-hard way that happens frequently in commercial entertainment.
for now, though, Fox seems inclined to treat the games as separate. and that might be true in terms of narrative, but Toby Fox is a musician and they must have therefore have an instinctive feeling for how two different works will play off against each other. they may be telling a wholly distinct story in Deltarune, but the expectations of Undertale fans will modify their experience of the new game; there will be resonances and interferences. UT fans develop their own ideas about what Frisk is like, what Alphys and Undyne are like, and so forth, from their impressions of Undertale; they carry those expectations into playing Deltarune, where Alphys and Undyne are different people and Frisk is missing algother, but there's someone named Kris who's eerily reminiscent of them. surely Fox is attempting to play with these expectations.
I think I've already given my own personal impression—I think Fox intends us to regard Undertale as something like a dream that Kris has woken up from, and maybe we'll get no better explanation than that. how often do you ever get an explanation for your own dreams? for us anyway it's basically "never". our dreams are murky to us, and yet clearly they sometimes reflect back bits and pieces that are recognizable; familiar faces pop up, or familiar locations. it may be enough, in Deltarune, to imagine that Kris had some kind of elaborate fantasy-themed dream in which versions of his friends appeared...an experience which may not mean much on its own, except that Kris is starting to have a LOT of "dreams" of that sort.
it's possible that the experience of getting really into a video game or other work of fiction is something like falling asleep or having a dissociative experience or something of that nature. temporarily you're in a different world, but then comes some point where you're interrupted (or you tell yourself the dream's over) and poof, it's gone. human beings, at least the human beings I've tended to run into in the particular slice of whıte American culture that I've been compelled to inhabit, seem to want that from their experiences with fiction. fiction can be learned from; fiction can be instructive and have direct real-world resonance. but the American habit has been towards pure escapism coupled with the glee of destroying illusions. "Star Wars", say, can be the most important thing in the world—until the moment it's necessary to declare "it's not real, it's just for kids, only [slurs] take Star Wars seriously" in order to get along in daily life.
people do that with Undertale of course. they enact a dream of pacifism—or a dream of mayhem—and then pop the bubble. it's not real. it's just fiction. it's just a game, or at best a moral fable nobody's obliged to heed. the sleeper must awaken, &c.
maybe Deltarune, in the end, is Toby Fox's longer and more carefully constructed reminder that some dreams do have consequences.
~Chara of Pnictogen