/jp/ - Otaku Culture

The stages shown off so far are Takane's stage in 18 (pic related) and Sekibanki's stage in 14 (seen in op's image), which implies that they'll probably be playable unless ZUN is throwing stages in at random.
It's also worth mentioning that >>43250609 screenshot's background is from 14.3, Day 6. Day 6 starts with a tutorial fight with Mamizou, and features Aya, Hatate, Nitori and Momiji, so any of those are possible (though Mamizou is less likely than the other four given her minor role in the stage, and if Takane is in I wouldn't bet on Nitori either). It could also serve as Seija's stage, if she were to become a playable character. Too many possible characters that could call it their home stage at the moment.
Their stages showing up, of course, isn't a solid confirmation that they'll be in the game, but it's worth considering when discussing possible characters.