ok ok ok i have a theory potentially discussing the answer to that, and it has to do with exactly why they are so similar
length of time since it’s been opened
we know that the Castle Town Darkworld has been around for a VERY long time, as Ralsei has explained and also as not had any lightners in it for at least since its conception. But we don’t even know if there WAS any lightners in its conception, Ralsei certainly hasn’t mentioned any, and while he isn’t a very reliable source for what HASNT happened, he has repeatedly reinforced that this is the first time he’s meeting ANYONE, including probably the creator. Which means this darkworld has been contained from outside influences for a very long time.
on the other hand we have the Shelter Darkworld, which has also been around for a long time and which we also do not know it’s creator. But this Darkworld is different due to its large list of lightners in it: Dess may or may not be a full or semi full inhabitant (Dess!Knight theory would support this but even if she isn’t the knight, if she is alive she is most likely here [or under the lake but stfu—]), Carol visits often, Kris visits often, whatever other Goners that may or may not exist have a full or semi-full time home here, and as the chapters progress we’re getting more and more kidnapped people: Undyne, Asgore, and potentially more people relating to the Pine Tree, Police Badge, and Delta Rune (which has the most room for interpretation at this point, it could either represent the church as Susie pointed out, which means if Alvin disappears we all know why, or what the Delta Rune represents, potentially the Creator of the Castle Town Darkworld)
The main juxtaposition we know of physically is that lighteners are EVERYWHERE in one and NOWHERE in the other. The only lightners that have ever been in Castle Town (aside from maybe the creator) are Susie and Kris and soon Noelle. While in every other darkworld there has been Kris, Susie, The Knight, Noelle, Berdly, Toriel, Undyne (although that was for a VERY short amount of time), Gerson (although he is debatably a lightener consisted his… alive status… Ralsei does still refer to him as a lightener [or at least doesn’t correct Susie when she calls him that]), and finally Asgore. From their inception they have has the Wills of lightners influencing them, potentially more literally if the Castle Town Darkworld doesn’t have a creator (or at least one in this reality, *cough cough* gaster *cough cough*). And in all the dark worlds with a lot of lightners (basically every dark world aside from Card Kingdom and kinda sorta maybe the Dark Sanctuaries), the darkners seem obsessed with lightners.
And you may think “oh does this mean Ralsei is being corrupted by Lightners Wills, since he’s around them a lot more” and NO because Ralsei is starting to develop his OWN Will, most likely related to the fact he is a Pure Darkner, and therefore incorruptible by Lightners influences.
And the reason Darkners petrify when in other darkworlds is because the idea of them is incongruent with the idea created of this new darkworld. Lancer can’t exist in Cyber World because it’s like throwing a Fantasy character into a Cyberpunk world, they ARE both fictional, but they are too fundamentally dissimilar to work. It’s also VERY inconsistant the rate that this happens, which of course doyalist explanation is because toby can do whatever he wants, but it may be because the ideas fit better. Lancer, while being a fantasy character you see on a tv show, is a fantasy character you see on a TV show. And the fact that there IS cards in the living room, or were, with the darkners in mancountry. Meaning the Will of the Lightners influence what kind of world it is, but in a Dark fountain divorced from the Will of any Lightners for so long, they are allowed to be all in a messy, incongruous, Town.
With this as the background, i propose another theory for how the Shelter will be different from Castle Town, which is the complete opposite. Where Ralsei is either the only Darkner that can exist in that world, or the only one that has any semblance of autonomy. Darkners in other darkworlds are seen with some kind of autonomy, just with a desire to help lightners. If there are darkners in the shelter, they may be wholly dependent on the lightners that inhabit the darkworld, doing everything they say with no questions asked. That would of course pose an extreme opposite to EVERYTHING we’ve been working up to in terms of darkner autonomy, and create an interesting situation for Ralsei (as you’ve said, there’s a strong possibility Ralsei would be VERY important in this chapter), because this is what he WANTS to be. Mindless, Will-less dolls that serve their lightners completely. That’s who Ralsei thinks he should be, but he’s so clearly NOT. He can’t even be affected by the Wills of Lightners in such a stained fountain like he SHOULD BE.
It would also create an interesting situation for Noelle, who even in the darkworld with the most lightners so far, she still got dragged around. She and ideas of autonomy go hand in hand, and putting her in a situation where people actively cannot help but bend to her Will would be interesting (especially with weird route implications, god weird route implications would make this place so fucked up, if we get it in chapter six then it would be a “heyyyyy remember the autonomy you DIDNT traumatize two teens and maybe even kill them and also traumatized another teen and maybe even killed ANOTHER ONE to get? remember how you DIDNT do that? you DIDNT do that, right? well HERE IT IS <3” but if it’s in chapter seven then ig we know what side b is gonna look like, which would be an interesting mirror to chapter two and ALSO an interesting mirror to chapter 5 in the way that it mirrors a previous chapter and borrows ideas from it to expand upon the questions and themes is asks and explores)
Susie would fucking hate it, if Lancer is in that chapter then she would very quickly make sure he is NOT. Because that’s her best friend, with ZERO autonomy whatsoever, sure he acts pretty similar (because that’s what she, a lightener, wants) but it would be like the second dark sanctuary, an uncanny valley. And Kris would have potentially interesting reactions to it. Obviously in part they already know the twist of the chapter, but we don’t know how much they interacted with the Darkners of the Shelter, so seeing their reaction would be… interesting.
And if this is true, then Ralsei would get completely ignored by the other darkners. Since he’s not important. He can’t join them in being under the completely and utter Will of lightners, and he can’t boss them around since he is still a darkner. So they don’t care about him. He might even be kicked out of flighting because the darkners give that little shit about him, not even enough to fight him even if a lightener asked. He would be like a ghost. Which would solve the four players problem, but also it would piss Ralsei the FUCK off. He’s fine being ignored by lightners, he’s fine being used by them, but as soon as a darkner does it, it pisses him the fuck off. Which would be interesting character for him.
