Has anyone noticed that the first thing you encounter in the caves of waterfall are plants and you need to interact with them in order to progress. And echo flowers (representing the flowers telling Asgore everything the people are saying).

This means flower king will be a dark world that’s of a darker color scheme reflecting Asgore’s damp colored room.
And then you go to the festival that’s winter and Halloween themed.
And then you go to the ruins (shelter).