Of all the character that I think appear, Doremy is the least likely of them...Though the implication of her appearance is interesting.
As Touhou's fan, you should already know about the "Butterfly Dream": Is it that we dreamt about the butterfly, or is the butterfly dream about us? Since Doremy is involved, it make me think that the possession mechanic is entirely (or partly) based around that, in which 2 people are sharing a same "dream", so to speak, and as such they share the same "reality". One person could be "asleep", watching the action the partner take as in the dream, and vice-versa.
...This mean that, if my conjecture is true, then pile upon spirit, dream is involved also, making the theme of an impossible dream more complex. Moreover, this could finally connect the "Humaned Away" incident with this, finally suggesting that the newhu would also be the perpetrator of that incident, or at the very least exploit that.
Edit:...Also, if Doremy is involved, then it could also be possible that Eirin would also be playable too... I don't know, just remember that Eirin does make a "Butterfly dream" pill.