WARNING: Spoilers ahead for Deltarune, if you haven't played the demo yet, feel free to do so and then come back later. You have been warned.
So it's officially been almost a week since Deltarune’s demo came out, and there’s a LOT to say about it. I've seen a lot of theories about the story of the game and how it connects to Undertale, pointing out plot holes and inconsistencies between the two.
According to Toby Fox officially, Deltarune is based in an alternate timeline from Undertale, based on inconsistencies such as Asriel being alive and Kris not being Frisk among other things. While it is possible this is Toby's own Undertale AU in response to fan works such as Underfell and Underswap, I like to think otherwise.
Going by the ending of the demo, my personal interpretation of Delta Rune and it's connection to Undertale is that the game is set in a post-Genocide route timeline, thus making both the Pacifist and Genocide routes the canon ending. All the events of Undertale happen as they do: Frisk falls into the hole at Mt. Ebott, meets all the monsters, fights the giant photoshopped eldritch abomination, and frees the monsters.
So what's changed, what happened to Frisk, and who's this person named Kris that's taken their place? Well, the answers are rather simple, at least in my eyes.
Placement in the timeline - It's implied that 8 years have passed, as stated on Toriel's computer in the school, and while the events of the first game are alluded to, they're not brought up because it's been 8 years, so everyone has moved on with their life from their time underground. There are plenty of easter eggs in the town at the end of the demo, some which are harder to find than others, but we know from them alone that the events of original game still happened, albeit with certain things changed.
Kris - Kris is an amalgamation of Frisk and Chara, much like the Amalgamates in the True Lab, but created under a unique circumstance. When Frisk sold their soul to Chara at the end of the Genocide Run, it was for the purpose of bringing back Asriel for real as Frisk went through the True Pacifist Run before doing the Genocide Run, and then went through everything again, but as a new entity known as Kris.
If one looks at Kris' sprite, he has the same skin color as Frisk and the same outfit as Chara, as well as the reflection Frisk has in the puddles in Waterfall during the Genocide Run. This clearly hints that Kris isn't purely Chara, but a fusion between both entities.
Undyne and Alphys - Undyne claims she's never met Alphys because it's possible they had a disagreement and broke up. Undyne doesn't seem to be upset about it because she's trying to control her temper about what happened between the two. Alphys has also branched out to western animation such as taking interest in a Simpsons-like animated series.
As for Undyne's eye, Alphys managed to give her a new one when they were still together. Interestingly, Undyne is now a cop, which she was also a cop in the Undertale fan-series Glitchtale after leaving the Underground. Chances are that's just a coincidence, but who knows really.
Mew Mew Kissy Cutie 2 - In the Librarby, there's a billboard with a review of MMKC2, which Alphys hated in the first game, and instead it states she thinks it's better than the first one. It's possible that her tastes have changed now that she's on the surface and has managed to be in contact with other MMKC fans who have different opinions on it than her.
Sans - He knows EXACTLY what's going on, he just doesn't care because he's Sans. That being said, that doesn't mean that we can't still have a bad time with him... *Megalovania plays in the distance*
Connections to W.D. Gaster - [DATA EXPUNGED]
Monsters, Lightners, Darkners and Ralsei - Ralsei is an anagram of Asriel, and we know he's a goat like him, as well as a prince. I have a theory that Monsters originally came from the Dark World long ago to settle in it, and became weaker due to magic working differently in the Light World. Ralsei is not Asriel, but he is part of the Dreemurr family, albeit a different branch that stayed in the Dark World. Lancer may call Susie a "Lightner", but she is more of a descendant of the Darkners that came to the Light World long ago, an event which may have led to the Monsters being sealed Underground.
The King of Spades is also similar to Asgore in the fact that he hates the Lightners just as Asgore wanted to destroy humans once he had all 7 Human Souls, though in the case of the King of Spades it was more of a case of xenophobia while in the case of Asgore it was a case of him wanting to free the Monsters from the Underground.
The Delta Rune - While this is the name of the game, the concept and symbol originated in Undertale, albeit with a different meaning. In Undertale, it's stated by Gerson that the symbol is ancient and predates the time the Monsters were banished underground to the point where it was forgotten what the symbol originally meant.
"That's the Delta Rune, the emblem of our kingdom. The Kingdom... Of Monsters."
It should be noted that in this particular line of dialogue from Gerson, the words "Delta Rune" are highlighted in yellow, and it also should be noted that this is the only time the Delta Rune is mentioned by name in the entire game. When first asked, it's implied that the Delta Rune is a typical "Coat of Arms", which are the emblems of countries in the real world, sometimes being the crest of the royal family in some monarchies, the Dreemurr family being no exception given how all three of the Dreemurr's bear the symbol on their outfit. When asked what the emblem means, he says this:
"That emblem actually predates written history. The original meaning has been lost to time... All we know is that the triangles symbolize us monsters below, and the winged circle above symbolizes... Somethin' else. Most people say it's the 'angel,' from the prophecy..."
Note how he mentions it predates written history. As for how the Deltarune game itself interprets it, it's obvious the Darkners remember what the emblem meant, as the three triangles represent the three heroes of that world's own prophecy. This is obviously what the original meaning of the symbol is, and that it isn't about the relationship between humans and monsters... though it could be interpreted as such with the humans being the top two triangles and the monsters being the bottom triangle. If anything, the symbol could be interpreted as one thing or another, depending on how it's viewed.
With all that being said, is Deltarune really set in an alternate timeline as Toby Fox says? It is... from a certain point of view, as Obi Wan Kenobi from Star Wars would put it. The only thing we're certain of is that it's connected to the events of Undertale in some way, and it is implied that the game's events still happened, albeit with a few alterations based on certain circumstances. Granted this is just the demo of the game, and that things are subject to change as to how the two games connect, it's obvious that some of my theories might be either modified or touched upon once the full game is out.