I had one of these last night, which is weird because I don't usually remember anything I dream, or if I do it's really mundane. I woke up very early this morning after dreaming this and wrote down a bunch of notes, so much of this is based on the notes.
In the dream I was playing the extra stage of an un-specific Touhou game for the first time (maybe Touhou 14, doesn't really matter). The playable character selection was huge, way larger than the cast for the main game. Based on the character portraits it seemed to take place many years after the current canon. The human characters were visibly very old, likely in their seventies or eighties, and many of them had signifiacntly different outfits. Reimu was conspicuously missing, Marisa was a very old woman, and Sakuya was similarly old and wearing an outfit like a stage magician instead of her maid clothes and had a sword cane. Youmu was an adult (although not elderly like Marisa or Sakuya) and had five or six sheathed swords on her back, a bow, and a quiver for some reason. Some of the youkai, like Alice and Satori, looked exactly the same while others looked significantly different. Yukari was more gap than woman, with significant parts of her body overlaid by gaps, and Reisen had a lab coat and stethoscope. I cycled through them a couple of times looking for Reimu and selected Marisa when I couldn't find her.
I don't remember much of the actual gameplay and I don't think it featured heavily in the dream, but I do remember that Marisa's sprite was wider than it was tall because she was sitting side-saddle on her broom, which was parallel to the bottom of the screen. The stage took place in the Bamboo Forest (I'm pretty sure the background was just stage four of Imperishable Night again) and featured an extremely short stage portion followed by three short midboss fights in a row against three other characters from the playable cast. The boss fight was against Ex-Keine.
After every spellcard there was a significant amount of dialogue, but it was in Japanese so I didn't understand it. I won easily because apparently in my dreams I'm good at Touhou and elderly Marisa seemed to be able to mow down enemies basically instantly (I believe her shot type was Reimu's Youkai Buster from Mountain Of Faith, but with big yellow stars). I think part of the boss fight was that the patterns never started at the boss, but rather started from all four corners at the screen at the same time and closed in on the character's position. I watched the ending and took a guess at what was happening, then looked up the script on the Touhou Wiki.
Apparently the story was that all of the characters received letters inviting them to the Bamboo Forest for another test of guts, and after one of them beat the rest down they would find that the letter was sent by Keine. Over the course of the battle she slowly reveals that she had been the secret mastermind behind Gensokyo since her childhood and that it was originally created by a real Hakutaku who passed it's power on to her at the end of it's life so that she could be Gensokyo's author. The Hakurei shrine maidens never really existed and were a fiction created by Keine and her predecessor to explain alterations that they make to Gensokyo's history. At this point Keine is at the end of her life and is trying to find a worthy successor to her power, so she gives Marisa her were-Hakutaku curse and vanishes. The ending features a shot of a horned, elderly Marisa holding a scroll, then Marisa teaching children at the human village's school, and as far as anyone knows she has always been there and Keine never existed. Then we see the new Hakurei shrine maiden, who looks like a cross between Reimu and Marisa and is flying on a broomstick.
So that was pretty weird. I don't usually have dreams like that. I think that was my brain saying "Hey man, you gotta stop playing Imperishable Night right before you go to bed". Fuck you brain, you'll get intense visual stimulation right before you go to sleep and you'll like it!