I thought about Ten Desires for a while and thought that the song name "Raise the Flag of Rebellion" implied that Mamizou was a big part of the next game. This has been said already. I don't know if the next game is Hopeless Masquerade, or Double Dealing Characters. Now you might say why Hopeless Masquerade is one of my options. It is, because Mamizou was a big part in the game. And there was a sense of rebellion in the fighting and tournament. I felt like it was rebelling that Gensokyo had no religion, so all the religion leaders tried to make Gensokyo believe in their rituals and ceremonies.
If it isn't Hopeless Masquerade, then it should be Double Dealing Characters. Double Dealing Characters also has that rebellion vibe. Except it is much more obvious and is the main problem, unless the corrupted weapons are the main problem. We still don't know which one is the main problem and which one is the side dish. If the main one is the rebellion, I feel like Mamizou is the final boss. If the rebelling problem is just a side effect, then Mamizou can be the fifth stage boss, or the extra stage boss. I think that she would qualify as a fifth stage boss if she is included in the final game.
Anything I get wrong or you disagree on?