I'm toying with this idea -- I posted a bit about it in the old thread, it's this cyberpunk-style story set 1000 years later, in which the Hakurei border is seemingly gone and youkai walk openly in the "Outside" World (there's nothing for it to be outside anymore). It would open with Mokou, arrested for the murder of Kaguya Houraisan; in the ensuing conversation with her jailers, it becomes clear that she is innocent, but her captors have a marked disinterest in her actual guilt or innocence, let alone the fact that Kaguya got up and walked away right after it happened.
We soon learn that Yuka Kazami basically rules the world from this Shinra-style corporation with a semi-iron fist, not outright beating people up in public with masked troopers, but sometimes, people ... disappear. This and other negatives aren't widely broadcasted, and she isn't feared exactly, but her YK emblem kind of makes people uneasy. It isn't all bad, though. Kazami Co. has completely and permanently blocked off passage to Makai, Jigoku, and other "dangerous" realms (more a PR move than an actual danger), and crime is ... well, it looks like it's down.
The biggest thorn in her side, now that she has captured Yukari Yakumo, is this thousand-year-old youkai-magician named Marisa Kirisame, who's been waging a private little war on Yuka's empire, along with her Kappa Corps (which contains several other youkai and humans -- unlike Kazami Co., it's an equal-opportunity employer). Yuka has been trying to brand Marisa's group as a terrorist, but only half the news stories about her tries to sell it, and only a quarter of the populace buys it. Yuka's defense is usually thorough enough that Kazami Co. and the Kappa Corps don't actually fight very often, but Yuka and Marisa have more intellectual struggles.
Marisa's ultimate goal, her biggest target, is a giant reactor Yuka built. The temperature at its core is usually around 7000 kelvins and never drops below 6561, but remarkably, the central chamber seems to require no coolant beyond low-level regulators. Why? Because of Kazami Co.'s best kept secret -- the youkai hidden inside the reactor's core. It is this youkai whom Marisa intends to free, and this will form the crux of the adventure. Along the way, we'll have a few visits to the moon, a large-scale assault with Makai's doll-army joining the Kappa Corps, a Thirty Xanatos Pileup or two ... The works.