Tell you what: if Rumia, Yuuka or either of the Watas appear in this game, I will write a work of fiction at least 800 words long depicting you being honored by the 2hu of your choice.
I personally do not think that characters have to be part of the religious conflict to appear in this game. Which is why I did not rule out either Rumia or Yuuka. This is mainly because for Yuuka, she is the only character in the PoFV cast that is not in the background.
For Rumia, the odds are less likely, because well, fighting game characters in story mode do not have characters that are "weak". Hisoutensoku being the exception in the sense that it wasn't really an incident, but just characters wandering around. This is my reasoning for why Meiling is unplayable in both Immaterial and Missing Power and Scarlet Weather Rhapsody story mode.
The Watatsuki sisters, I do not think is likely at all, because I don't see why they would appear in Gensokyo.
To post my opinion, I don't see why people think there should be 2 Moriya characters. Reason for that is because they are Shinto(ism?). And due the fact that for now there are just two people from Buddhism and Taoism, why they should be 3 from Shinto? Or did you people forget Reimu was Shinto Shrine Maiden?
Reimu is a Shinto shrine maiden, but she doesn't fight for Shinto. Her affliation is the same as Marisa, neutral.
If Reimu vs. Reimu is possible, then it's possible for playable character to be a background character at the same time. I can imagine a Reimu vs. Reimu match at Hakurei Shrine, with a Reimu in the background either watching the match, or being annoyed with everyone else. In other words, if there can be two Reimus, why not three?
If you go by that, being in the background does not mean unplayable, which is a possibility, then, any character is playable.
But right now, if they are playble, they do not appear in the background, which is why lots of people take this stance. That said, none of the playable characters appear as a background character, even if they should in some stages.
Also the more surprising part is that the Moriyas look like they are not playable, assuming you take the stance that BG=unplayable. That means the roster just opened up for other characters.
I would like to see Akyuu as a playable. I have hope, because she has yet appear and she did not appear in the Human Village or Byakuren's temple, where I expect her to appear. But since she travels quite a bit, she might appear anywhere, including the final boss's stage.