We'd get a hybrid platformer/danmaku game where there will be six stages, and even numbered stages play like a Touhou game while odd-numbered stages play like Cave Story, the extra stage combines elements from both. The plot would be that a mysterious floating island appears over Gensokyo and Reimu and Marisa go to investigate. It ultimately ends up with Reimu and Marisa teaming up with Quote and Curly from Cave Story to fight a resurrected Doctor. (in which a bored Yukari and Yuyuko may or may not have been involved with...)
The stage bosses would be:
Stage 1 - Cirno
Stage 2 - Sanae (Because the Moirya shrine
has to stick it's nose where it doesn't belong...)
Stage 3 - Misery*
Stage 4 - either Tenshi or Iku (Since the island is floating pretty high, it should draw
some attention from Bhava-agra...)
Stage 5 - The Island Core* (Danmaku variant, to make it even harder...)
Stage 6 - The Doctor*
Extra Stage - Ikachan* ('cause why the hell not?)
*originates from one of Pixel's games.ZUN and Pixel would also do a joint music score, which would lead to some interesting tracks.
Pixel would make the dialogue art/mugshots, since we all know ZUN art is "the best" art...
What if we got a Youkai based off
Charybdis?