As one gains experience, being able to adjust to a game's genre or parts of its system is unmistakably engaging, and I think that games without that spark are missing the point. ... The way I see it, however you decide to establish a world, you need to decide on the the design of the created world. During the establishment of a game, particularly when making characters for the so-called world creation, people make the mistake of saying "This won't have any effect on the game", but even among these people there are those who say "Because this character is in the game, I hate it." This claim is proof enough that even characters can influence a game.
I'd say this more or less hits the nail in the head with regards to the Touhou fandom, among other games. In another thread here, someone said, "If you're playing the game for the character, you're doing it wrong." I'd disagree. The game is more than the mechanics. It's more than the music. If all people were interested in was the mechanics, shmups and bullet hell games would consist solely of white dots on a black background. Because a developer puts time into creating a world and a cast of characters with backstories, personalities and interpersonal relationships, the game is a lot more fun and engaging.
As far as the question of "it's fundamentally impossible to create something like Touhou with more than one person" is concerned, I admit I've never designed a game, but I think there's some truth to this. In other creative endeavors, such as a novel, for example, or a television show, there's a tendency for the plot to meander and branch needlessly, for inconsistencies to arise, when there's more than one writer. Of course, other pairs of eyes reading over what that single writer has done is essential to making a finished product of quality, but the actual writing should be a singular endeavor to maintain focus, and consistency of plot and feel.