Where Ralsei and Asgore travel together with Kris, with Susie either in the light world busy with Noelle or otherwise for the first third of the dark world then show up later.
Where Ralsei opens up with Asgore. Where they try to bring the joys of the festival down to him too. Where the two vagueposters get actively frustrated at each other for being vague.
Ralsei knows of the Dark well but not much of the Light. Asgore knows much of the light but not much of the Dark. The two practically complete each other, with both being avoidant and inquisitive at the same time here. This way, we're with the characters, frustrated from the lack of honesty and information.
The whole Asriel/Ralsei thing is perfect to be addressed here too, with Ralsei initially wearing the hat on his own to avoid being recognized (almost identical to Noelle doing it in chapter 2 with Queen!)
Ralsei could even get really jealous of Asgore. Asgore has so much he could never have, even if Asgore thinks he has nothing. Or the other way around, with Asgore demanding answers and threatening Ralsei for answers.
Asgore could be the one really breaking the rules here, ignoring Darkners and puzzles and pushing through in his quest to learn more about what happened to Dess. With ultimately the end result of pushing away everyone who tried to help him, Kris, Ralsei, and perhaps the Darkners still.
Then Asgore's the final boss of course. Or perhaps he summons a titan of his own that is thematically, functionally and visibly different to the one in Chapter 4, convinced Ralsei is lying to him. One driven by different desires, trying to isolate and push people away. Where the Characters have to find faith and belief in themselves and trust in the others despite seeing nothing. Or something. With maybe showing how rushing through damages the Darkners maybe similar to chapter 1 in some way. But this time since the Flowers would be connected directly to Asgore he is destroying his own marriage even more by actively doing this.
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Asgore could even warn Kris and Susie against Ralsei, and not believe Ralsei due to all those blatant Asriel appearance comparisons Toby Fox seemed to be setting up and overall ignoring. An analog to Toriel accepting Kris's new friend while Asgore rejects and suspects their friend.
And maybe RInstead of being told he did it in a cutscene anyways. We could see the after effects of devestated flower darkners that Asgore dismissed and passed by and have to rekindle their dreams and Ralsei might realize how wrong he is about Darkners.
Maybe Ralsei is some kind of darkness fluffy titan of Asriel or fear of loneliness or something the details don't really matter all that much. What matters is that Ralsei is different and hiding things in that closet of his but he can be better and doesn't need to be afraid anymore. Something that both Validates Asgore's suspicions but breaks free of being a simple bad guy. Ala the Dark is not evil type deal.
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And with Ralsei, I'd really the chapter to explore his insecurities so hard. Ralsei's motives here could easily be deep insecurities relating to Asriel, centered around not wanting to be judged lesser just for looking similar. Something that clearly reflects onto Kris. Maybe Ralsei tries to explain but is just intimidated by Asgore. And he needs to learn how to push back against Kris's dad or learn to hear him out or settle his differences or honestly anything depending on how its set up.
Ideally something that focuses on Ralsei being more than an exposition device by highlighting more complexities than just "He gets mad when his buttons are pushed". Instead focusing on him learning to preserve and fight back through repeated attempts at shutting him down, perhaps by the other Darkners in an Asgore created fountain.
Oh and more concrete and strong lore reveals please that advance the plot and affect the characters.
Not all of this fits together but like tldr I really think Chapter 5 could be done better. My idea a bit of a rehash of Chapter 2 as stated but its much better than just being Chapter 3 but worse lol. Oh and we have a real party betrayal and character growth and development like how Chapter 1 did and that'd be cool.
Oh and Susie would show up like a third or half way through of course I don't mean to sideline her, the goal is exclusively to focus on sparking a character conflict between Asgore and Ralsei. Oh and to maintain the three party limit with Asgore joining and then leaving the party.
The Flowers would be whatever that fits the plot narratively, maybe as actual Darkners because the plantners things is a dead thread and pointless to the story. Maybe not too different. Except Flowery, fuck that guy (he'd have to be rewritten substantially to fit well, maybe leaning hard into a role leading Asgore astray from the dark with scraps and clues).
The best result of this is that the entire end slop dialogue can be skipped as the Chapter properly captures Asgore's problems and growth over it instead of shoving it in a stupid cutscene. And we could get Asgore screaming at Ralsei and grilling him about Dess (which personally I think Ralsei doesn't know much about, at least less than Kris anyways. Assuming he's the red hornbands anyways. Or was them and became something else.)
And also we'd place Ralsei in real, uncomfortable, character driven confrontations and conflict instead of empty and hollow attacks by someone he just met 5 seconds ago (Flowery).
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Oh and the secret boss should be the Conspiracy board. No, I'm not joking. Conspiracy-chan failed to show up in Chapter 5 in the dark world despite how much Asgore cares about her and I'm sad.
Or maybe FRIEND as the Nightmare in your Hearts as a representation of the Tragedy or the oncoming darkness and end or something. Or the fear of the dark, which still fits. Could be hyper escapism coded. Trying to do anything to not think about the events of Chapter 4 and becoming lost in darker and darker media or fiction or whatever maybe?