Touhou 15 is a pleasant surprise - although I will note that I totally called Reisen being playable last July. If it follows the pattern I wouldn't be surprised if Kaguya, Eirin or Tewi showed up as a boss or midboss, but the possibility that the Watatsuki sisters might show up instead is interesting.
Having Kaguya, Eirin or Tewi as bosses/midbosses doesn't seem to fit to me, given Reisen's description seems to imply that Eirin sent her. Of course, that's only what I personally interpret from it, could be wrong.
I don't like the looks of it, to be honest.
No playable Youmu is ZUN's worst decision. The images are also hard on the eyes with those bright purple colors. It's like I'm tripping on acid. ZUn saying he'll experiment with the system and whatnot does not bode well, considering the recent trend (by which I mean the trend of the last 3 years or so). I can only pray.
You've got to be the only person I've seen so far who reacted negatively, lol. I like Youmu a lot, but no Youmu is fine by me if we get Reisen instead - at least, it's not just Reimu, Marisa and Sanae. That would have been boring. Hell, it introduces a new playable character in main games, even better than Youmu or Sakuya again. Youmu has already been playable in a main game before afterall. If anything, Reimu, Marisa, Reisen and Youmu/Sakuya would have been nice.
As for experimenting, the way he announces it sounds to me like he'll massively alter the gameplay, unlike other main games. Could be good or bad (hopefully good, of course), but only time will tell. DDC was a disappointment, but it certainly wasn't because of experimentation.