Okay, time for a little prediction time with next to no founding whatsoever, maybe? Really, there isn't much to go on anyways...
First, this game is obviously about spirits, ghosts, the netherworld, Youmu and similars, sooooo... I'm awaiting the glorious return of the Saigyou Ayakashi. Maybe someone else is trying to bring forth the soul the sleeps beneath... or is trying to bring the tree to full bloom again? There ARE cherry petals floating around everywhere in those screenshots.
Second, tying possibly in on the first but also inspired by someone somewhere mentioning the word "Necromancer" in this thread. Maybe someone from the outside world finally managed to followed her friend into Gensokyo and started up an incident to lure her out?
Third, monkeys. Just because I feel them somewhat fitting to the theme of this game and they are an old part of Japanese mythology that's getting lost (which is usually ZUNs basic root for things that appear inside the border). I literally see before me three monkeys, one blind, one deaf, one silent jumping from the branches of the fully bloomed Saigyou Ayakashi.