Huh, weird. My computer didn't update today's messages, so this one is for the Hitler What If.
The Yin Yang tank would come back, only this time as a Panzer. Hitler would ride atop, and he would be the Stage 6 boss in a massive Incident in that spans all of Gensokyo. Heinrich Himmler would be the Extra Stage Boss.
Also, lots of lasers. And a swastika danmaku pattern.
What if Reimu was the demons?
I assume you mean youkai, and not demons like Shinki. Anyway, it would be a story about how a youkai herself was a youkai exterminator. Not much more that's different, since Reimu herself has been described as a youkai, I think.
Edit: Thanks Lincoln. What if Danmaku became a sport in real life?