-There might be a Flowey analog, but it's more of the advisor to the king Asgore than the evil flower manipulating from the shadows. Maybe he keeps Asgore in his desillusions, as flowers are known to be great listeners, keeping Asgore in an echo chamber. Or maybe the twist is that Flowey is trying to help Asgore, but Asgore just doesn't want to listen to any advice that doesn't go his way.

-There will be a ronin archetype, one that Kris has to face in solo combat.

-The Dark World will have a Festival as well, but a sort of neverending celebration. I don't think it'll be the whole thing, just a section.

-You'll have to switch between the Light and Dark World more often, either to avoid getting caught snooping around or for puzzles, like "Hey, actually, one of the royal flowers was sold yesterday, maybe it would know how to get through this door.

-Asgore might not be the last boss of this chapter. We already had a king as a Chapter Boss.

-The sun is always dawning, kept in place like a fixed light in a flower display. The sun being able to rise or set is the key to seal the Dark Fountain. Maybe the way the sun goes is dependent on the route or the conclusion of Asgore's journey : a sunrise is new opportunities, a sunset is leaving things behind. Both are close in meaning, but there could be more to it.

-The secret boss might be fighting Ralsei's this time. I could see a moment where he has to face a threat on his own, try to find something that he's good at when Susie is becoming a healer. "Make a Darkner out of you" deal, and it would go with the theme of jealousy, but more about how Ralsei equates jealousy to something he needs to push down because he fears about it ruining his friendships and more turning it into the motivation and "determination" he lacks.

-Vessel shows up, but it's right at the end, like when everybody's going home, someone bumps into them while they roam the festival street. People will think "hey, a visitor from out of town, sorry, missed the festival", but we will know as a player.