All right, full post about the concept now:
This could actually be a very nice game that combine various gameplay from the mainline game. For example, there's a series of "scene" that are related to the UFO incident, so during those scene you have to play around UFO token to get through (whether aiming for score using blue one, or surviving use red/green, or even trying to break as many as you can). Another example would be the "season" incident, where you have to use you're season release to survive/score as many point as you can. You could even have "survival" challenge where you must last a specified amount of time (much like PoFV). I am getting hyped just thinking about the possibility, image this one: Cirno having to use her GFW ice power to protect an ally (that doesn't move, so there's no annoying random). Or having to use "cherry border" to survive deadly spells (collecting cherry item + unfocus shot to charge it).
Having smaller incident also make it easy to justify having a larger cast, and I like the idea of having only some character available for some scene. It can play pretty well into the story, as well as offering some variation into the gameplay. I do think the game should force the player to try out each character at least once, in a kind of "invisible tutorial" way where you force the player to do ~2 scene, but the first one is easy and builds of the character strength (ex: Youmu can take out multiple "strong" fairy in a single charged slash), and the second increase the difficulty a bit using the character's weakness (ex: Youmu's has to get closer, so the player must now learn when to get close, and when to get away).
Regarding accessibility, I think simply having difficulty setting would be enough. Currently, this is what makes the Photo games inaccessible to newcomers (only ISC is good since you can "choose" your difficulty depending on which item you use, or not). Maybe give the option to increase the amount of lives for longer survival-scene / boss-scene, or have broze-silver-gold ranks for score-scene.
Such game would take a lot of work, depending on the number of character, scenes and difficulty/rank. One can dream (thanks for sharing that dream).