Yea, IaMP brought about one new character, SWR brought two, HM brought one, and ULiL introduced one new character (not including Kasen)
So one character is most likely, although two is possible as well (but by now there are so many other Touhou characters to draw on that I'd be really surprised if there were resources to spare for a second new character rather than the devs deciding to bring over one more existing character to the HM engine and style)
(I'm not counting Yorihime as a new character, because even if she does show up, I highly doubt she'd be the culprit. Thus even if Yorihime is in the game, likely as the sub-boss akin to Yukari in IaMP, there would probably be a new character to be the culprit)