Okay so, Touhou Fuumaroku: Story of Eastern Wonderland was the first shoot-em-up/bullet hell game in The Touhou Project. Six games later, came Touhou Eiyashou: Imperishable Night, and with it, the final boss Kaguya Houraisan, explicitly stated to be the character "Kaguya-hime" from the fairy tale, "The Tale of The Bamboo Cutter", otherwise known as "The Tale of Princess Kaguya". Six games after that, Touhou Kishinjou: Double Dealing Character added in Shinmyoumaru Sukuna to the cast, who is stated to be descended from the one inch boy, Issun-Boshi, from the fairy tale of the same name. We're all aware of this, right?
Now then, a theory. To my knowledge, the big three fairy tales of Japan are "The Tale of The Bamboo Cutter/The Tale of Princess Kaguya", "Issun-Boshi", and "Momotaro". Since that last one hasn't had any representation in Touhou yet, my theory is, that the final boss of Touhou 20, what it will be, will have a final boss tied to the story of the peach boy. What connection, I dunno. Maybe it'll be the oni, giving a tie to the story and naming the final member of The Big Four of The Mountain.