I've got a very ambitious idea. At the core it's a massive crossover between Touhou, this little-known strategy game called Dominions, and a -lot- of other media. I suppose I'll have to explain Dominions first.
In Dominions, various pretender gods fight for true ascendancy via the nations that worship them. These nations are based off of mythologies and cultures from every place and time in human history: there are nations based off the Aztec and Incan Empires, the Roman Empire, Ancient Greece, Imperial China, Eastern European folklore such as the Fae, African Tribal Confederations, even the apocryphal Jewish writings. Power levels reach insanely high, with the mightiest mages able to conjure up second suns that make the world unbearably hot, plunge the world into eternal night, accelerate time so that all living things die from old age in a few years, call creatures from the most nightmarish landscapes to serve them, etc. etc. That is to say, the horrendously overpowered Touhou characters will not feel out of place among the horrendously overpowered beings of this setting.
Gensokyo itself is dropped into the world. Rather, the inhabitants of Gensokyo now perceive the "outside world" to be the world of Dominions, and anyone may cross the border by passing beneath the gate at Hakurei Shrine. This change happens without warning or explanation, and the story starts shortly after that. Some will ignore it, but the barrier shift causes problems for lots of people - Reimu, Yukari, and Eiki, for example. Others, such as Kanako&co., Yuuka, and Yuyuko&Youmu cross over for their own reasons, some more permanent than others. I don't want to get too deep into the plot here, but there will be a massively epic plot. That's the plan, at least.
Other crossovers, why? Because there are some very interesting characters I've experienced in other media that have room to develop, and some aren't even in the fantasy genre (they are, however, still really powerful). And since basically all media has some cultural/mythological influence, I can drop them off in a region which corresponds to that influence. Gensokyo, for example, gets dropped off near the Japanese-and-Chinese-based region.
Original characters. Not a favored thing, and I understand that. But these OCs are designed to be able to stand up on their own and only tangentially interact with existing characters unless it serves a real purpose. I disdain self-insert Gary Stu OCs much as the next guy, so I won't write them.
Lastly, character development. This isn't just a story of "Touhou characters beat the shit out of various outsider people/monsters/gods/C'thulhu's", although there will be a good bit of it. Rather, I want to give many characters drastic development via this extremely unusual occurrence. It's hard to explain. But as they say, good stories are character driven rather than plot driven.
I would greatly appreciate any feedback on this idea in the thread, but I'd rather not divulge specific details here. Writing has been commenced, but it goes slowly. Email me for additional information or suggestions.