I had this idea running in my head, but never wanted to do anything with it.
A massive multi-direction babel-like black tower appears in Gensokyo, which going up, breaches the heavens, and the the top of the great border causing a massive black hole in it.
The lower half of the tower reaches through Underground, and past that breaching the border further into the cattle-grounds, and feeding grounds where the INCREDIBLY unfriendly youkai reside that aren't allowed up topside that we've only heard about in brief conversation by canon.
The tower has a final protrusion going off to the direction of Makai, the entrance located in the Mountains from the Hakurei shrine.
Due to the Tower, what powers command it, or what lays within, Yukari has been forced into her slumber out of season, and is unable to assist.
Additionally, Reimu has been experiencing heart pains since the tower's appearance, presumably they will worsen the longer the tower breach of the great border continues.
The story will have three Heroines take on the tower in three alternating routes, upper, lower, left, and finally the EX-stage, The Bottomless world, made accessible on completion of the other three routes, and identification of the EX-boss.
To summarize the individual incidents caused by this multi-incident tower:
-The Breach into Makai is causing it's residents to migrate to Gensokyo freely once more, and is also causing Makai's native miasma to spill out into Gensokyo, which has begun to kill things off.
-The heaven breach is causing new deities, and their servants to manifest due to it being closest to the outside via the Great Border breach, and it's causing a ruckus because the whole incident has shaken local faith, it's a mess up there as well down on the ground.
-The Lower breach has made it's way to the Feeding-grounds, a containment ground for the worst of the worst Youkai that pay no heed to rules, treaties, regulations, simply either by choice , or due to lack of control, and have also begun spilling out and attacking humans and starting fights, and of course are seizing territory over the cattle-grounds, where the 'unique' food sources supplied to surface and underground youkai alike are kept/shipped and received.
Additionally, it's caused some problems with the distribution of electricity over Gensokyo thanks to it cutting into old-hell.
And of course, the local shrine maiden and meddlesome youkai are also factors.
Due to the number of problems, most heroines are up top protecting humans, or performing damage control, and only three girls can go.
The places the tower breaches into are also impossible to access without going through the tower, so the girls must enter to tower to get to the according destination.
The girls chosen can only be playable characters from Canon Touhou Danmaku games.
Each girl may choose an assist character for their respective route.
The introduction will have the girls start off at their individual locations, doing their daily routines before the Tower hit, the aftermath, the team up, and the introductory stage where they fly up to the top of the tower to try solve things quickly.
Tenshi will show up to brief them on things occurring topside on her side of things, Aya will tell them of Makai, and (Insert underground resident here) for Underground and below that.
The heroines will discover they cannot access the top floor of the tower, and are greeted by a black shade with a red visage that openly spews some vague nonsensical BS as a type of 'magical per-recording' to it's motive and plans, hinting it is not in control of what is going on It's under possession/mind-control/whatever you wish
The Makai route has the chosen heroine follow the floors leading to where the long Tower corridor to the mountains and Makai itself splits off from the upper route, and will also be with the heroine taking the upper route for this duration.
Proceeding through the corridor to it's passage to the mountain cave where Makai is, Heroine will encounter:
Going up the tower:
Same bosses as upper route until the 3rd floor where they split up Socialization goes here, description of the interior of the tower, bunch of other shit.
Two bosses, inter dimensional bullshit goes in here, lore, short stories etc...
Actual Makai Stage:
>That fucking poison doll I can't remember the name of, is interested in the Miasma from makai spewing out
>An Escaping Makai youkai
>PC-98 girls, old & new
>Religious faction member
>Shiki as the final boss, possible religious faction member 2-ways assistance.
Upper Tower Route:
Up up and up until you reach heaven.
Beat up Tenshi anyway, old times sake
YHWH and Sariel show up, are up to dirty kike SMT bullshit, kick their asses.
Deity ass-kicking Montage saints also present.
Trouble in heaven.
Is generally the action route of the three routes, fun for all.
Underground, follow the destruction trail, run into a few old faces, meet Utsuho trying to not go Chernobyl on everything.
Cattle-grounds Explore darker themes of Gensokyo here, numerous humans and other special orders from suicides, or bred in captivity/under various magics to simplyfy though processes, get reel 2deep4u/feels animal farm here.
Feeding-grounds Unleash your inner edge here, we're talking post apocalyptic hellscape hardcore mode no visible hitbox action here, with devilman/Return of the overfiend tier grimderp up the ass. Time to kick omega demon ass and stop rebel scum, also gain insight into the nature of the Final boss and EX-boss.
Final boss is a notable youkai demon or the like know for a massacre or something on par with that, no mercy. Yuugi shows up to help against a particular boss she knew or something.
Throughout these routes you may hide insight into the final boss within things scattered amongst the tower and it's infastructure, and events in the story.
The Final boss, and culprit of the tower is The one referred to as 'The Vampire'
No, not the SDM's Vampires, as in the asshole who one day waltzed into Gensokyo, decided to play the Vampire chess kings and pawns game with everyone and nearly ruined all fun for all time, and had to be curbstomped by Yukari resulting in the food supply treaty made to shut them as a whole up
'The Vampire' can be anyone, can be a character based on an existing person, figure etc and characterized accordingly and can be male or female.
The only requirements is that 'The Vampire' must be under some form of control, must have had her weaknesses/history mentioned in any of the routes thus far and must be Type-Moon levels of broken as a vampire, also lacking what Flandre refers to as an "eye" to boot, and can only be beaten in a Danmaku battle, as rules dictate.
'The Vampire' has been using the towers various incidents to power a ritual of some kind, sending off souls, supplied via the deaths caused by youkai from makai and the feeding grounds, faith from humans received to the new deities that have arrived and a some-other non descriptive force of entropy resulting from the breaches caused by the tower, which acts as a pump that directs these forces to the ritual via a power on par with Vicissitude that works for abstract concepts, like being able to mold air and re-purpose it.
After beating 'The vampire' the heroines will incapacitate him/her and will Stop the ritual, which ceases the Towers functions, though not removing it from gensokyo.
This is the investigation period, in which the tower is explored with all three heroines to discover who the real culprit is, because the first culprit has memory loss from the control they were under and is confined to the SDM basement to keep Flandre company (not having an eye opens wondrous possibilities for her)
The culprit will reveal the real culprit if the players can figure out the name of the culprit based on the evidence supplied
The Culprit is
Who due to some reason you the writer will explain along the course of the story opted to give herself the same virus her best friend was subject to resulting in her slowly becoming youkai just as broken and old as her best friend, which had subsequent effects on her best friends other.
Long story short, for reason of revenge, jealousy, tragedy or long term really hot trans-dimensional yuri bullshit, Renko, by following her best friends dreams is subsequently unraveling Gensokyo itself to power her rebirth, as a Youkai either to her knowledge or not as a consequence, causing the 'The Vampire' by Renko's power/will, to act as the catalyst for her rebirth as a Youkai unto Gensokyo because 'The vampire' had the means and power to feed her whilst she developed as her dreams progressed, as Renko slowly became Youkai in the Same method her best friend did.
So Renko is being born as a youkai and is taking it Gensokyo over, destroying it, is unconsciousness to it all or conscious and this shit's really up to you and how you characterize her relationship with pre-yukari leading up to Viral insemination, shit could've been an accident for all we know, she could even be replacing or eating Yukari in the process of all this.
As the ritual has ended, she will begin to progress to wake up at the normal rate, which was nearly done anyway as Yukari wakes up, as weak as a lesser Youkai because any later and she'd be pretty much sleeping for good.
The Vampire drops the bomb and some vague shit mentioning the culprit knows Yukari's true name, so she can't pull her punches or plot-armour her way out of the fight or anything, and heroines drop what they know about the culprit, rising X suspicions in late morning gap-bitch
Yukari and the gang go off to the Bottomless world, which is the motherfucking bottom of Higan itself, or at least the lowest point of the endless river using Gap hax to actually make it so one could fly/swim through it all.
There are three bosses at this point, The Yama, as turns out this location is forbidden to be accessed under old-hag treaty and bitches ain't got time for that shit, the fuck mothering Saigyou Ayakashi or at least a minor aspect of it, as the shit 'The vampire' was siphoning into the ritual included bits of that old sealed thing, now in delicious kimono wearing pink haired blue-eyed with small branches coming from it's shoulders shota form! and finally Renko herself, born at the base of what appeared to be a massive vain, but was actually the shredded skin of the great Dragon itself.
Dialouge, then the greatest Danmaku battle in the history of ever ensues. there are two forms, nearly born Renko and Born Renko complete with her own umbrella and broken ass to match gap-bitch powers on an even level due to knowing her true name,
Ending is up to you, make sure to remove the tower though.
Best yuri couple ever conceived umbrella hag marriage ultimate border of tribalism and scissoring end is preferable.