This is a non-C.Bird centric post but I just wanted somewhere to share this idea I've had for a while that hitched off of a strange interaction in deltarune chapter 3
So right after reaching the Rouxls Kaard boss room, returning to the stealth room you came from there are now two miniature goulden sams called "goulden son"s. They only serve as fast travel to get to the end of this really long room, but also have a cute sequence of interactions. The first son you talk to wishes there was a goulden son "2" and sends you to the other side to look for one. Upon finding them and relaying the request of the first son they promptly oblige and dub themselves goulden son 2. You can then return and inform the first of this development. However, if you go to goulden son 2 they now have a similar issue where they wish there was a goulden son "3" because 'it's hard being the only son with a number in their name'. You head to the other side again and tell the first son about this which results in them changing their name from just "goulden son" to "goulden son 3". Again you can inform #2 about this and everyone's happy
Now where I'm going with all this is it's hard not to look at this interaction and think just what exactly was the point of all that. The first thing you could assume from the names of these NPCs is that it's just a cheeky reference to the Golden Sun series, but I wasn't satisfied with that especially with the strange conversion of the first son into the third. Where I went with this was that it was alluding to the chronological order for the games Undertale and Deltarune.
What might be confusing about this at first is that there isn't a third game outside of these two to use for this scenario, and while that is true it's also true that there are only two goulden sons in the previously described interaction. What I propose is that one of these games, just like what happens with one of the goulden sons, goes on to become this "third installment" of the series. Before I convolute things any further I'll layout the order plain and simple. It goes
Deltarune
Undertale
Deltarune
Story-wise it's not that hard to believe if you really start to consider it. For sans (and more importantly Papyrus) to ever play a role in Undertale's events, he had to have already escaped whatever world ending event that happened to the place on the surface he once called home. Most everyone has surmised that this 'place' is what's shown in deltarune and that what sans had to escape from was the Roaring.
So from these assumptions, the events of deltarune had to have happened before Undertale, truly allowing it the title of "UnderBound 2" (which toby "radiation" fox called it in his post on starman dot net) By that notion we can appropriately call the events of deltarune that sans survived simply "UnderBound"
This all still begs the question(s) 'what then makes deltarune also this hypothetical third installment?' 'shouldn't all this sement the game as purely a prequel?' well that hinges on your suspension of disbelief and how truly real you perceive the events that occur in Undertale and Deltarune to be.
A large aspect of Undertale has to do with acknowledging the player and focusing on the fact it is their every action that determines the events of the game; never faltering on showing the lasting effects any run, reset, or even death has to this story. While the effects can be very minor, they still work to paint this picture that Undertale isn't just one story that occurs in a vacuum, but rather an all-enveloping experience which occurs to the player. That is to say, Undertale isn't comfortable presenting itself as just a game, but instead a window into another world that exists alongside ours albeit in a more vulnerable and mailable way.
The presentation of deltarune and the releases of its chapters work even harder to reinstate this notion. First two chapters of deltarune were announced on social media not by Toby himself, but this mysterious voice that's assumed to be Gaster, a character from Undertale. The implication of this being that, Undertale's characters could still go out to do things on their own to later affect our reality, even after the game's story was over. This is made more than plausible because of the true pacifist "ending" where Flowey begs the player not to reset the game afterwards as that would prevent Frisk from getting to live out their life.
Of course the status of Gaster lore-wise is dubious at best. However, the prospect of the player's/soul's/angel's time travel shenanigans creating whole new instances of this reality leaves the door open for there being a reality where Gaster shows up just to make contact with the our world and get us injected into the events of deltarune. And that is ultimately what I'm trying to get at when I propose deltarune as also being this third installment to the series
After all, if sans says there was no point in trying to go back to his world during the events of Undertale, but we're now able to do exactly that when playing deltarune and we're expected to perceive these games as another reality which experience the passage of time even without us observing it, then how am I not meant to believe that us being able to experience deltarune in the first place is a direct consequence of the player/soul/angel touching down on the events Undertale causing disastrous anomalies that indirectly provoked a once average scientist turned interdimensional entity to reset a doomed world just to see what more chaos could erupt from us messing around in there too. It's not unlike a scientist to want to probe around in places they shouldn't for the sake of some newfound greater knowledge or small discovery.
And to touch back the whole point of 'the only reason deltarune was restored to the way it was before the roaring is because Gaster reset it': if you're having a little trouble believing that's actually the case then I'll remind you that when you type out his name it resets you back to the beginning. Most people just associate that tidbit of information with the fact he's this elusive individual that didn't leave a trace after his disappearance, but to me if Toby really wanted to drive that point home he would have just had the game crash. Not resetting the intro sequence. And when the game did used to crash on consoles after typing the name out, they acknowledged Gaster for once and patched it out
So yeah deltarune is technically both a prequel and a sequel. There are some places in deltarune and Undertale where this is alluded to and it even ties back to the RGB occurrences with blue, red, and green corresponding to the first deltarune, Undertale, then the second deltarune. The color placed in the middle signifies what game is currently being played, that's why they're mostly only BRG occurrences in Undertale (mushroom puzzle answers in the ruins and Papyrus colored shapes order) and RGB occurrences in deltarune (a b & c rank doors and Noelle's blog post ornaments)
Anyway if I were to give this theory a name I'd go with "dalet rune theory" as dalet just stands for delta in a different language but is still an anagram