This story never truly ends...

I think the sequence of events outlined in this tweet is pretty likely: Susie is not great at reading the room, and she's (as of the end of chapter 5) totally unaware of Rudy's condition. Despite her baseline knowing the dark worlds are risky business, the end of chapter 5 has her more immersed in the escapism of it than ever before. With the sense of escapism and her being caught up in her budding relationship Noelle, it wouldn't be out of character for her to make a hasty introduction.

As for Noelle: from what we can infer from both routes, she feels trapped, weak, and helpless in her current reality, a condition that will likely be on full display at the start of chapter 6 with Rudy's failing health. Now, I'm far from the best at predicting Noelle's actions. But regardless of how the whole thing goes over, if she's exposed to 1) dark worlds being real and 2) healing being real within them, it's pretty likely that she will make a dark fountain, and make it within the hospital, in order to help Rudy.

She'd lost her sister, and her only remaining buffer between her smothering mother and herself is her dear ol' dad, who by all signs is not doing well (Rudy in Undertale being deceased, him being sick and pushing himself very hard to set up the lights to make Noelle happy). At the festival, despite saying he's "feeling better", he's still coughing and clearly in fragile health, something even more obvious in the weird route. Whether he Fell Down or simply fell down, he's likely not in a good state right now.

It's a long shot, to think creating a dark world could let her heal her father. In both routes, she is part of a conversation about doing "something crazy". While this could just be a parallel of normal and weird route, that doesn't mean it can't also hold significance for the story progression itself.

Still, why Noelle? Why not the Knight, or Carol, or even Susie or Kris if they're dragged along on this?

Chapter 2 shows Queen deliberating over the process of creating dark fountains, with Kris, Susie and Noelle being shown in the sky. Granted, Berdly could've been exempt because she doesn't consider him a notable Lightner, but the order of headshots and the fact this graphic exists at all seems like foreshadowing. Along with the entirety of chapter 2 being her trying to make Noelle create another dark fountain.

The hospital has been in the background of a lot of happenings in the game, and is one of the only major locations that's thus far been mostly optional to visit. Yet it has significance, both in housing Rudy, a character that's very likely to be chapter 6 relevant, and (if you take the egg rooms, specifically chapter 4's, as significant) a place that Kris also has some history with.

Rudy's accident is what separates Noelle and ends the Suselle fun. Flowery is pretty obviously meant to imitate Rudy (similar hairstyle, expression, pet names for Asgore), and he's all in our face only moments after we're exposed to Rudy's dire situation, leaving him in the forefront of our mind through the starting portion of the dark world. Toby also likes foreshadowing bits about the upcoming chapters in little ways, and I think this oddly specific battle flavor text in chapter 5 is one such instance.

So, with Rudy "falling down" and assuming Noelle will indeed be introduced to dark worlds, the dark world being opened by Noelle and it being done in the hospital seems like the most likely course of events.

Perhaps the most interesting question though, how does this benefit the story, and the current arcs?

Point one: Convenience.

We're approaching the eleventh hour, with Toby keeping a lot of major questions close to chest.

Chapter 5 has repeatedly asserted that Dark Worlds are like dreams, and it's heavily implied that the will of the Lightners within it have a major influence on how it will present. This is speculation on my part, but if Rudy is convinced that it's a dream, he might walk through it, exposing a lot of the secrets Kris and Carol have worked so far to hide from the player (and themselves).

Point two: Setups.

I've already mentioned things from chapter 2, 4 and 5 that seem like setups that have thus far had no clear payoff. Chapter 5 especially has given us a lot of teasers and new information about The Incident, a lot more than before (which was almost nothing), but still not enough to get a full picture. Due to pacing, it seems like we're on the cusp of getting at the very least some more critical information.

We've also been shown that healing moves can miss, presumably if the injury is too grave (but we don't really know as of now).

This chapter has also amped up on certain character-relevant themes: Noelle's desperate situation, Susie's wanton escapism, Kris's powerlessness and self-hatred. It seems like all three might come to a head, where normal-route Noelle will finally "do something crazy", Susie will be faced with her own hypocrisy in trying to talk her down from it (paralleling Ralsei's own foil situation of ch5), and Kris feeling increased strain between trying to keep their Knight Duties and Hero Duties separate (which has already happened, in the ch3 Roaring Knight fight, and has potential for reprisal).

Also, at the end of the chapter, Asgore comes to realize his wrongs, and makes plans to actually make up for his mistakes. But he does so a bit too late, as the knight had already opened a second fountain, and he's taken by the Knight. (This is a tool that will help us later)

Point three: Story

This point will be more inherently speculative, so let's take stock of what we do know: the story is approaching its conclusion, Toby has implied it will be a turn for the darker, that the Fun Adventures are over. Chapter 4 was dark, and heavy on story reveals. Every chapter that featured Noelle also gave us pieces of her past with Kris. It's not unreasonable these trends will continue going into ch 6.

I've already talked in the previous points about how this presumed set of circumstances is ideal for the boiling point of several characters' arcs, and fits with Toby's trend of late-game being heavy on reveals.

For the sake of keeping this tidy, I'm keeping those reveals to just what has been implied as important in the normal route: the Incident, more information on Dess in general, and the final shelter code. Just these three could make for a meaty chapter, that also has room for character development alongside lore.

Now, here comes the fanfiction writing: my rough idea for what the chapter could look like, taking all this into account.

Susie approaches Noelle. Maybe when she's outside the hospital, getting some air, to "show her something". Cue her introduction to Castle Town, and being faced with the Dark Worlds being real, magic being real within them, and either freaking out or excusing herself in order to go to the hospital and make a dark world there.

Within the Hospital Dark World, Noelle pursues Rudy, who assumes this is a dream, or his life flashing before his eyes, and us seeing the timeline of events. If Carol is there and pulled into the Dark World, we could get a lot more context on their arrangement, and a lot of tension between Kris trying to keep their two alliances separate, being faced with their own memories of Art Therapy, and with Susie's focus square on retrieving Noelle continuing her dismissive streak towards Kris's distress at the information exposed.

Perhaps Noelle just uses the dark world to spend a bit more time with her dad. Or, she struggles to keep pace as he wanders through his memories, the dark world creating distance where there previously wasn't any. And when the time comes, she tries to heal him, only for it to miss, again and again, and once the fountain is sealed all that's left is his dust.

We are already familiar with how dark Deltarune can get, and that factor is amped up with every chapter. I think the chapter will not have a happy conclusion, and that Rudy is by all available information destined to die. If the outlined circumstances (Noelle-made hospital dark world) come to pass, these two could easily coincide.

Some observations from my friend @commanderfreddy that I hope speak for themselves

I don't think it'll be a happy goodbye, or a fulfilled goodbye. The dark worlds are Escapism, which while good in small doses, ultimately prevents you from living your life to the fullest. It drags you away from what's real and important, something pretty explicitly shown with Asgore in chapter 5.

Maybe it's all too much for Noelle. Maybe the set-up of chapter 2, of her creating a fountain within a fountain, finally comes to pay. And maybe that's when the roaring finally happens. Maybe the last piece of the prophecy comes just in time for chapter 7, the final tragedy unveiling.

Last observations (dubious, but they might become relevant)

One big question is the Lord of the Hammer, which is vague about chapter 6. But with Susie's white pen of hope, and Gerson talking about the younger generations taking up the pencil, "the author being swallowed by his own creation" feels like it may be alluding to a rather grim turn for Susie's character.

She's been acting odd all chapter. Starting with determination towards the shelter mission and with a lot of attention paid to Kris, but gradually losing the plot as she gets swept up in the moment and Noelle's attention. The story is testing her, her role as the mold-breaker, (no ribbons, not being willing to stand the status quo and predetermined path) and she's starting to lose points. I don't think her arc necessarily leads to her sinking into the rock bottom of escapism, but akin to Asgore she decides to come up for air just a little too late.

I hope it's not controversial to say that "the second hero" and "the girl, at last, was the girl" are two separate entities within the prophecy. I think Noelle and Susie are vacillating on the roles, Susie currently being the second hero, and Noelle being The Girl.

Still, Susie believes herself to be both. And she behaves odd, almost like someone who thinks they don't have a lot of time left.

(chapter 4, in the last light world segment after seeing the prophecy; she says it unprompted)

(chapter 5, while holding Noelle at the lake)

(Susie talking about the prophecy after witnessing it, implying a preventable tragedy, possibly with direct involvement by the heroes)

As she grows complicit with Her Mold, and perhaps by the prophecy by proxy, she incidentally becomes more open to ribbons, something that could have mixed significance depending on your interpretation of Susie's GNC status, but it's worth noting that Ralsei has a negative reaction to it.

It could just be him being petty, considering how she wore it at Flowery's prompting but not Ralsei's, but still. It stuck out to me.

It seems like the final prophecy might foreshadow a terrible fate befalling one of the two. Still, we can't do much with this information right now, because it's assumption upon assumption since the line between her and Noelle's roles in the prophecy are intentionally blurry, and will likely remain blurry for a while longer.