The crew arrives at the festival. There's some sort of minor blowup as expected resulting in either Noelle or Susie storming off, leaving you with the other. Then the Toriel, Asgore and Sans encounter happens. Asgore gets super butthurt and storms off, Your plus one says 'uh shouldn't we go check on him' leading you going to the flower shop and bam it's a dark world for some reason, cue you having therapy time with Ralsei as you do stupid colored flower door puzzles. Pretty sure Asgore isn't going to be the main villain though, he's just there and histrionic and winds up being the bonus boss (as per ch4 vents eavesdropping dialog) with the spurned party member taken hostage/brainwashed by the knight or something as the real chapter boss.
The only real curveball I can see here that would make me happy is if Ralsei was nowhere to be found if you were on the Noelle route implying whatever Ralsei is in the real world is being carried by Susie rather than Kris and could lead to some really fun setup for later chapters with Ralsei not knowing what happened.