Chapter 5 will be 7 dark fountains made during the festival, creating a "megafountain". One for each flower, and each flower will be a darkner miniboss.
Purple flower will be in the Town Hall which will become a courtroom Phoenix Wright style minigame. Could also be Police office.
Cyan flower will be in the Holiday house and become an ice world.
Orange flower will be in ICE-E's pizza/QC diner, a firey hell dark world.
Blue flower (foreshadowed by Miss Mizzle) will be in charge of Sans's store/Papyrus's house dark world, secret boss will be fought here and use the blue soul.
Yellow flower will be Flower King, jungle/forest world.
Green flower will be hospital, Rudy plot happens here.
Gold flower will be town plaza where Asgore fight + actual villain + Roaring happens.
All these dark worlds will be much much shorter than the regular dark worlds but they'll all connect together and most of the townspeople will get involved including Catti and Jockington.