For information, I divide word between the time before and the time after I read Toby Fox's message on Deltarune (which I did just after he released it)...
BEFORE:
I've kinda forgot about some points actually that tend not to be a sequel. My first thought on it was a bit too speedy and I jumped to quickly. Sooooooooo what I was thinking at then was a "parallel universe", a little bit like Majora's Mask is to Ocarina of Time. You know, oddly similar-looking characters, that you have already met in another universe, but not here. So that Deltarune is a parallel universe to Undertale, where you meet the same characters... but it's not the same ones. What happened in Undertale did not happened and will not happen here. Not only because they don't seem to remember what happened (but for Sans, I wouldn't trust a single word from him anyway), but also for one single person: Asriel.
We all agree that, no matter what path you chose, Asriel is DEAD in Undertale. Is it by killing flowey or he surrenders and vanish in the pacifist run, Asriel died in his youth. So the fact that he is in College now, it's impossible that it is a sequel... but also that it cannot be a prequel! If he died in his youth, I can't have been in college, can he?
So, just for Asriel, only a parallel universe would make sense. Also for Ralsei (again an anagram with Asriel), just by looking to this image, I understood that the dark world was only their imagination (or anoooooother parallel universe).
Finally, a friend of mine gave me an interesting theory about this WHAT THE FUCK ending. Remember what Chara said at the end of the genocide path? He was going also to attack and destroy other parallel universes/dimensions (I don't remember clearly the word he used). That being said, may it be him, taking over Kris and locking him up (his SOUL anyway) in the bird cage? That would also be an interesting thing...
Oh boy, too much mind blowing with maybe false theories.
AFTER:
So that's tend to confirm the parallel universe theory, although he doesn't want to draw the connections I made in mine. Voluntary hidden thing or me going way too far? Can't say it and won't say it for now.
I entirely agree with releasing the whole game at once, when it's finished. I wouldn't it like divided with several chapters to purchase separately.
However I regret one major thing: the unique path. Some already noticed that a genocide run in this first chapter seems impossible, so then only an 100% pacifist run is possible, making the battle system half-useless, if you don't use the attacks. I think it would lose the charm Undertale had, when everything changes depending on how you were going in the game...