I'm personally more interested in what Youmu's shot types are gonna be like. One's probably gonna resemble her shot in Imperishable Night, but that still leaves one or two others.
I'm surprised that she's getting a playable appearance, but at the same time I think I kinda expected it. Since she's half-human, I suppose she fits the necessary requirements.
Just speculation, but this also might be the start of a Hakugyokurou-character arc in the overall series. EoSD more or less got one (Sakuya's playable appearance in PCB, as well as both her and Remi in IN, along with how the inhabitants in the SDM got a major role in SSiB), and both Cage in Lunatic Runagate and Silent Sinner in Blue were direct sequels to IN. On top of that, you might said that Subterranean Animism and Hisoutensoku are both direct sequels to Mountain of Faith as well. Personally, I've been wanting more focus on the Hyakugyokurou gals for a while now, so the prospect does kind excite me.