I still vividly remember the feeling of discord that seized me upon my first playthrough of "Deltarune" Chapter One upon its release, a feeling that was much amplified by a weird glitch that attended our first launching of the "Survey Program": everything ran at a glacial pace, both the initial character-creation fakeout stuff and the subsequent game, and it took about a second for Kris to make a single step. I was somehow taken with the feeling that this was deliberate, as if Fox were intentionally giving us the experience of trying to control another person through a very low-bandwidth hookup, and we played for a couple hours that way, getting as far as the first Dark World scenes, until finally restarting the game and getting normal behavior after that. Now that was an accident surely, but it reinforced the feeling of oddness and offness about that first Deltarune experience.
Right away, Deltarune Chapter One confounds expectations set by Undertale. Kris gets much less freedom of action in their early scenes—Frisk gets to run around in any direction right from the start, but Kris's Hometown experiences are practically on a rail (and Fox rubs the point home with the introduction of Susie.) The musical score of the game resonates weirdly with Undertale's music; if I knew the slightest thing about music and music criticism, I'd be able to describe the feeling better. The simplified graphics of the Chapter One Dark Worlds, which feature some occasional deliberate teasing about their paper-backdrop artificiality, feel oddly incomplete in comparison to Undertale—I remember wondering, on the first playthrough, whether they'd be upgraded when Deltarune was finished.
Somehow the feeling of discord with Undertale is worse in Hometown. Everyone you meet feels like awkward, uptight versions of their Undertale counterparts. I found myself thinking of "Mother 3" and how Tazmily becomes warped and creepy under the influence of Porky Minch and the Happy Boxes; even the tinkly pianola music throughout Hometown felt somehow too old-timey-sounding and vaguely threatening, like it was ironic counterpoint in a horror film. These odd feelings faded somewhat as we played Deltarune more and got better attuned to the different vibe of the game.
Toby Fox is an accomplished composer and must surely have a keen appreciation of discord in a musical sense. Not all sounds combine in a way that's pleasing to the ear; not all pieces of music sound well-matched if played one after the other. Thus I suggest that Fox intended for Undertale and Deltarune NOT to fit together. The most salient thing about the relationship between the two games, I surmise, is that they're not related. They are designed to clash with each other.
Overzealous fans have a bad habit of wanting more of the same. That's also a neurodivergent thing. We are so used to feeling at odds with the world that we cling too hard to those few activities which seem genuinely pleasurable, doing them to the exclusion of all other things, like Ikari Shinji keeping up his repetitive cello exercises because he liked doing it and nobody told him to stop. Fox knew the worst excesses of fandom (q.v. "Homestuck") so I don't put it past them to make Deltarune as deliberately upsetting as possible to all expectations established by Undertale, especially the expectation that the two games are somehow continuous with each other and fit into a grand design. Of course I still have hopes that Deltarune will somehow explain Undertale—what Undertale fan wouldn't?—but if my guess is correct, Fox has no intention to do so. Deltarune parallels Undertale but remains a separate fictional universe, so I believe.
Fox could upend all that speculation with a single announcement. Foxes are good at making trouble, aren't they?
~Chara of Pnictogen
