I like how the plot this time is focused on youkai. By the way... since we got even a Rokurokubi this time (And I thought ZUN would not be able to make such an ugly youkai look cute, but he did!) Then I suppose it'd be cool to see a Baku as well. Manipulation of dreams/etc... it'd be some fun stuff.
This reminds me of a conversation I had with a friend about something like this a long time ago concerning baku and ungaikyo (This was quite a while before DDC was even known). It involved an old legend that dreams could possibly be a way of conveying the true self while nightmares are a persons fears and malice and that these fears can corrupt a person to commit evil. Though I'd think a baku could serve it's own plot based on Mizuki Shigeru?s Mujara.
Was talking about DDC with a friend and I can only hope that one of the bosses is an ungaikyo just from seeing the mirror on the title screen. Mirrors are said to reveal someone's true self or their deepest desires (what it reveals or does depends on the legend and the origin of the legend), so I would hope that it's an ungaikyo (based on bloody Mary or some other western myth about mirrors?) that is stirring up a person's deep and dark parts of their souls. Though that's just me being a hopeless dreamer. :3
Though I would probably think that an amanojaku, like someone said, would be a plausible choice (hoping for this too since Nurarihyon was my favorite manga) and all these other theories sound logical. I'm hoping that they are right so we can all bask in that person's moment of glory

On a different note. I love EVERYTHING about this game! The PoC gimmick, the music, the characters, and the shot types! Though I do think the game is WAY too easy, even on hard and lunatic. Still, this'll probably end up as my favorite game with Kagerou (found her to be pretty easy except when using Reimu A) topping my list of favorite characters next to Okuu.~