A thread on Resetera has broken down the details of a new line of dialogue that was added into the Nintendo Switch version that suggests more Undertale content is on the way.
The fans of Undertale knew that some new content was going to be added to the Nintendo Switch version of the game, as a new character named Mad Mew Mew was shown in some of the Japanese previews. This follows on from the new additions to the PlayStation 4 version, most notably the Dog Shrine.
In the old Undertale versions, there is an NPC in the Waterfall area who is known as Clam Girl. She talks about her neighbor's daughter, who is a girl named Suzy, and how the protagonist should meet her.
You never meet a character named Suzy in Undertale, or at least no one who identifies themselves by that name.
In later patches of the game, talking to the Clam Girl will create a new line of dialogue when you search the first drawer in Sans' Workshop. The new line refers to a picture of three poorly drawn people and writing saying "don't forget". This same patch will also give the Clam Girl a new line, where she says that Suzy might be the reason why the protagonist came there in the first place.
The Nintendo Switch version of Undertale has added a new event when talking to the Clam Girl. If you return to the Waterfall area during the epilogue, she will say that "The time to meet her...is fast approaching" and turn grey, before disappearing.
Characters who are grey and vanish have been linked to W.D. Gaster, who is a secret character in the game's files. We still don't know much about Gaster, except that he seems to be a skeleton (and possibly related to Papyrus & Sans) and was the royal scientist before Alphys.
So, what does all of this mean?
The fact that Clam Girl talks about a "fast approaching" encounter with Suzy suggests that new Undertale content is on its way. This could take many forms, such as a prequel, sequel, or even a side game that takes place at the same time as the events of Undertale.
One possible theory is that the reason why the protagonist first ends up in the monster realm was that he was looking for the girl named Suzy and that she somehow got involved with the W. D. Gaster storyline. The fact that people seem to forget things involving Gaster may suggest why the protagonist forgot about his quest over time.
It's also possible that the line is nothing more than a way of baiting the fans, which wouldn't be out of character for Toby Fox, as he has included many hints and secret messages within Undertale that seem to only exist to inflame fan speculation and theories.
Will the story of Suzy be told in Undertale 2 or Undertale Zero? Will the Suzy tale be part of some upcoming DLC story for the game? Was the addition of the Clam Girl's new dialogue just a way to get fans talking about the game again, now that it's out on a new system? Only time will tell...