So far all of the chapters of deltarune have mirrored undertale, just going backwards. I didn't come up with this, this is the YouTube theory I saw that suggested it.

While that video was made ahead of Chapter 5's release & predicts it will mirror Snowdin, now it seems that chapter 5 continues on from Chapter 4 in being reminiscent of Waterfall, with the various plants & reeds surrounded by water, fields of grass the player can hide in, an antagonist (Flowery, Undyne) who while initially presented as untrustworthy/intimidating is ultimately sympathetic and is only fighting us to protect their world and the people they care about.

If this continues to hold true for the next two chapters, this pretty neatly slots into Chapter 6 mirroring Snowdin, and Chapter 7 mirroring the bunker (i think this would especially fit if this is the chapter where we go into the bunker, even if its not the main setting given Susie only has one code for sure at this point. Given that it seems pretty poorly maintained & has plants growing over it, it could fit a 'ruins' type environment well)

However, going back to Chapter 6, if the chapter does take place somewhere that mirrors Snowdin, the most reasonable place for this to be would be the Holiday home, already extremely themed around winter/snow/Christmas.

I also think this makes sense for the game for a number of reasons. Firstly, its really the only major spot in town (besides the bunker) that we have left to be a potential Dark World. Most of what remains is either minor character's houses or areas that seem like it would be very hard to make into any kind of Dark World (Sans' shop? the lake? maybe the hospital or police station could be contenders but those are kind of it just off the top of my head).

Secondly, i think this makes sense from the perspective of the main characters motivations as they stand at the end of Chapter 5. Susie has two clear goals at the end of the chapter: 1) bring Noelle properly into their team and tell her about the Dark Worlds and 2) find the codes to the Bunker to stop the Knight and rescue people inside. Kris meanwhile, for whatever yet to be revealed reason, really doesn't want Susie to get inside that bunker and stop the Knight.

So, I think a pretty reasonable plot with this knowledge could go:

At the beginning of next chapter, Susie sets out with Kris to tell Noelle about the Dark World (does Susie know what apparently happened to Rudy at the festival? Kris told her something but I can't recall off the top of my head how specific it was). There are then a couple different ways I think this could play out to get to a Dark World at the Holiday house.

The first option is that Noelle (who in the Normal route still believes what happened in Chapter 2 was a dream), is stressed out from what has happened to her father at the festival, whether that be a general fall/him being extremely ill or dying. As such, she gets upset with Susie, thinking she is making fun of her or otherwise tricking her by talking about 'Dark Worlds' and convincing her a 'dream' is real while she is going through a stressful time. Susie is herself upset by this, and decides that while Noelle (and presumably Carol as well) is at the hospital, she will sneak into the Holiday house with Kris to find the code in the guitar, get a step closer to opening the Bunker, and prove to Noelle what is going on. Kris, who firmly does not want this to happen, panics & to prevent Susie finding it creates another Dark World when they arrive at the Holiday home.