A large portion of the fanbase will scream in frustration due to the fact that would literally be a Touhou and Vocaloid crossover and we all know how much animosity there is that exists between the two fandoms.
What if Monsters, Inc. x Touhou?
I actually knew nothing of any animosity.
Youkai Inc, probably run by Yukari, is dedicated to making the humans scared so the youkai don't disappear in a puff of logic. Byakuren tries to be friendly and, if she succeeds at being friendly, everything collapses unlike in Monsters, Inc. Because, unlike monsters, youkai aren't meant to be thought favorably of.
This is why the spell card systems exist. Youkai can cause conflict, but it remains nonlethal and the shrine maiden can easily put a stop to it, everyone wins.
...I may have something wrong here.
What if Touhou x Harry Potter? Not sure if done before but whatever.