Deltarune is a sequal to Undertale (not really)

[Kyndle]

Possible spoilers by the way...
Since all of Undertale's endings are canon, and deltarune is set in a different universe with similar characters; there is no way that this could be a sequal/prequal (as Toby confirmes).
But in the end of the Genocide Run (reminding that this ending is also canon) we see Chara as they destroy the world, and they say "Let us erase this pointless world, and move on to the next".
Chara says this directly to the player -meaning you- and not to frisk. Keep in mind that Chara and The Player are two different beings and Frisk was the vessel we controlled throughout Undertale.
The theory is that after Chara (and you) erase the world you move on the the next one, The world of Deltarune. With Kris being your new vessel you control as the player, you are now in a counterpart to Undertales world.
The beings that could travel through space and time in Undertale were You -as the player-, Gaster, Sans, and Chara. We can hear Gaster's theme all thoughout the Dark World of Deltarune. And we can hear it in the normal town near the door we can't open yet. We see Sans in the town with seemingly no changes to his design and character whatsoever, also we know that Sans has the ability to Teleport, and when we visit Sans and Papyrus' house we can see Sans come out of a door with some sort of light/gas that changes color underneath it. Considering that we use the same door to teleport between places in deltarune it is possible that Sans can also visit the Dark World and exist in the normal town. What I'm saying is these characters are now in the world of deltarune just like you.
The only important thing about this theory I had is that You cannot choose who you are or what is happening around you. But maybe that just applies to chapter one and not to the other chapters. Then you might say that you might not choose to destroy the world of deltarune like you did with Undertale but we have the Soulless Pacifist ending for a reason, in Undertale you can still choose what you want to do even after Chara tells you to move on to the next world. You can still choose to recreate the world of Undertale since you are in control. But seeing Chara and how she decides to erase the world even if you say no might mean that this outcome is also wrong.
Maybe you can't choose what you are doing in Deltarune because Chara was in control the whole time. Think about it. You have to sell your soul if you want to Recreate the world of Undertale. And at the ending of the Soulless Pacifist run you see Frisks face turn into Chara's face. After the ending of Genocide run Chara gains so much power that now they are in control, and you have no choice but to destroy the world of deltarune as well, just because she wants you to do it. That's why you can't choose the outcomes of the events in the game.
As for Asriel, you kill him at the end of the Genocide Run and he's not a being like You, Chara, Gaster or Sans without the 6 human souls and the souls of all monsters. He was just the child of Toriel and Asgore who gained power with the souls he got.

But of course, as I said, this only aplies to the Genocide ending. I believe I covered the possibilities of the story of deltarune (wheter you can or can't control the outcomes of the events). Feel free to share your thoughts on this theory.

[Scott]

TLDR

[captainwiseass]

To steal a line from Jamie Lee Curtis, it's not a sequel, it's an equal.