Okay, so this is *definitely* just a crack theory that is most likely wrong, but what if in the last chapter of Deltarune we go into that red door at the end of the map, only to find that it is the Underground, but empty. Devoid of life.
Except the weird science man who lives in said cavernous, partially-lava-filled basement. For some reason. Idk, scientists are weird.
And therefore the last chapter is just us-the red soul, who has been to the Underground before, returning again once more.
This would make the Underground officially, finally go ‘empty’ by saving (or not) (hahaha insert Snowgrave joke here) the last Monster left behind? After all, if Gaster never left the Underground in the Pacifist Ending of Undertale, then that means the Underground was never ‘gone empty’, correct?
Therefore, Deltarune is simply a massive cry for help from the Goop Man himself to finally set him free from the Underground, fulfilling the prophecy stated in the original game that was never actually fulfilled in the first place (and also explains the religious themes in Deltarune if we are the Angel mentioned by the Priest Guy)
