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Mini Deltarune theories I have that are not quite big enough for a true post, but interesting to note. In order of most confident to least.
Ralsei is luring the lightners to the Dark World in order to trap them
In chapter 2, Ralsei creates rooms for Kris and Susie that "they can go to no matter what is happening outside". Unlike Queen's rooms, which are guesses based on search results, Kris and Susie's rooms are accurate to an unsettling degree. Not only is Kris' room identical to their light world counterpart, but it also has all of Asriel's trophies on their side of the room with Asriel's side gone, a reflection of their potential secret desire to no longer be under Asriel's shadow. Similarly, Susie's excitement on even having a room suggests that Ralsei could know what Susie's light world home life is like or lack thereof if she's homeless.
By creating these rooms, Ralsei is fulfilling the lightners secret desires and judging on his phrasing, wants them to go there in order to avoid whatever is happening "outside" the dark world. By doing so, he is effectively strengthening their desire to stay in the dark world so they can't escape.
Chapter 3 isn't going to be Undyne or Napstablook, it's Asgore
Chapter 3 taking place in Kris' home with the fountain being created by Kris suggests to me that the world may mostly involve whatever the Dreemur family drama entailed, which would include Asgore who was the former chief of police. Not only that, but the combination of Toriel having troubles calling the police and the station being closed in a preview for chapter 4 suggests to me that Mayor Holiday could have closed the station during the night of chapter 3 after they failed to catch the mysterious criminal wrecking Hometown, leaving Asgore as the only character left who might want to resume his policeman duties when Toriel's call is made. It is also more fitting for Toriel to kill Asgore in the hypothetical chance the Snowgrave route continues with her as she already dislikes Asgore and possibly blames him for Dess' disappearance. Speaking of...
Asgore really was involved
There's a lot of weird forshadowing on an "evil Santa". Lancer believes that "ho ho ho", a laugh associated with the mystical Christmas gift bearer, being an evil laugh. Noelle is also scared of Santa and it's revealed in Undertale that Asgore dressed up as Santa to give gifts to the monsters. All of this seems setup to me that Asgore really was somehow involved in letting Dess disappear, something that may be revealed during Chapter 3's Snowgrave route.
Dess is ruder than she appears
This is a more playing by expectations theory, however there's certain lines and implications that become interesting once looked under the lens of this theory.
It's revealed that Dess once beat Kris with a wiffle ball bat for scaring Noelle by telling her that ICE-E "eats kids". This puts a precedent that Dess has at least somewhat violent impulses in order to protect her sister, even in situations that don't need it. It's also a bit odd that unlike Asriel, no one in the hometown mentions Dess after her disappearance except for Noelle. Even Berdly, Noelle's best friend, doesn't bring her up. It could be that the hometowners may not want to hurt Noelle by reminding her of the disappearance, however since she's so open about talking about Dess to even new kid Susie it may be another reason why the hometown doesn't mention her. It's possible that the simularities to Dess and Susie (both are horror loving, somewhat violent, tomboys) are deliberate and are setting up the reveal that Dess is/was actually a bully and a future scene with Noelle may parallel the scene in Undertale where Asriel reminisces that Chara wasn't that good of a person.
Gaster blasters are named after Gaster, but not created by him
If you spare Papyrus in Undertale after an aborted Genocide run, he will say this.
The most common theory is that the "Special Attack" would be a Gaster Blaster. What's interesting to me is the phrasing "my Special Attack", which suggests that Papyrus owns this attack and it could have been unique to him before Sans or the annoying dog stole it. If Gaster Blasters are really his unique attack, it could mean that Papyrus created the Blasters and named them after Gaster, rather than the fandom interpretation that Gaster made the Blasters then gave them away.
There might not be a singular "Knight" because anyone can dream
At the end of each chapter, the characters ask if the dark world adventure they had was some sort of dream. Since anyone can fall asleep and dream, it's possible the game is setting up that trying to seal the fountains is impossible because anyone can dream and thus become their own "knight".