Update: Undertale developer Toby Fox posted this on Twitter, perhaps confirming an announcement for Undertale 2 or a related game:
For those who completed UNDERTALE it's really important that you check @UNDERTALE 24 hours from now.— tobyfox (@tobyfox) October 30, 2018
I want to make something new, and it all begins with your feedback.
A Windows PC / MacOSX PC is needed. https://t.co/N1IJLyiswy
Original article continues below.
The recent release of Undertale on the Nintendo Switch included a few new lines of dialogue that suggested either a sequel or spin-off is coming, or that a major new content update was coming to the game.
The official Undertale Twitter page has been putting up cryptic messages over the past few hours, telling the fans to come back in 24 hours for some kind of revelation.
RETURN HERE IN  HOURS.— ██████ (@UnderTale) October 30, 2018
The all-caps writing and talking about looking for someone has led many fans to suggest that this message is somehow related to the mysterious W. D. Gaster, who is a secret character that originally could only be found by hacking the game. It's now possible through updates to encounter Gaster (or rather, a figure that people believe is him) as well as his mysterious grey servants.
The rumors of more Undertale content started in September when fans discovered new lines of dialogue in the Nintendo Switch version of the game that was linked to a mysterious character named Suzy, who is never seen in the game. The new dialogue states that the time when you met Suzy was fast approaching.
So, what could these mysterious revelations be referring to?
The original Undertale was a huge success and created a massive fanbase of loyal fans across the globe. The game still has many unanswered mysteries, so there is plenty of material for a sequel.
The talk of a character named Suzy who is part of the original monster kingdom may mean that there is a new game that uses the original setting of Undertale, which may be set at the same time as Frisk's adventure or slightly before. This could offer us some info on W. D. Gaster, as the player may be around before his mysterious "accident."
Undertale In Smash Bros. Ultimate
There are rumors circulating the Internet about an impending Super Smash Bros. Ultimate Direct. Masahiro Sakurai has talked about Undertale in his column in Famitsu magazine and the game is huge in Japan, so it's possible that there may be Undertale content in the game, such as a playable character (possible Frisk, Sans, or Undyne), an assist trophy, or possibly a stage (such as Snowdin Town).
Whatever the case may be, we will only have to wait another day to discover the truth.