So one of the Japan aspects I wanted to emphasize were the religions, and do it differently than in other treatments like OA.
The Religions of Kaidan are Twisted and Broken (Kaidan is a Horror setting afterall)
Of course, I had to include my version of Shinto, which we called Yokinto (not a name I like, but we used), primarily because in Kaidan's prehistory, the yokai (animal based shape-changers) were the first inhabitants of the archipelago of Kaidan, before humanity by a thousand years or more. Yokinto shrines are located in places of outstanding natural beauty - waterfalls, mountain lakes, hot springs, unique large rocks in various landscapes, with local clerics as priests. The kami spirits consist of Yokinto heroes, nature spirits, and the ruling spirits worshipped by the imperial family.
Buddhism, which our version is called Zaoism (a kind of cross between Zen and Daoism,) because it's an existing major religion today, hasn't really gotten the love I thought it deserved. Now Zaoism is not Buddhism, there is no Nirvana, no escape from the reincarnation cycle - the cycle is all measures of the religion of the afterlife - of course that too is disturbed in Kaidan. The reincarnation cycle of Kaidan is broken, though it contains the 5 worlds/layers of the cycle: Heaven, Asuras, Human, Hungry Ghost and Hell. Because the Aristocracy, Office of the Shogun, the provincial Daimyo and the Imperial Court are undead from the curse of the setting's founding, Nobody except the aristocracy can achieve that caste - PCs will never be aristocracy, however the aristocracy is undead, thus do not participate in the reincarnation cycle. Thus the cycle is broken.
Strangely I tied the social caste system to the cosmology, so which social caste you are born in is cosmic. Now the social caste system only consists of 4 castes: Aristocracy, Samurai (Bushi), Commoners, Hinin/Eta (Oppressed). By Zaoism: Heaven = Aristocracy, Asuras = Samurai, Human = Commoners, Animal as an added caste refering to Yokai beings, Hungry Ghost = Hinin/Eta, Hell has no associated caste. Player characters accumulate Karma points, at the pace of Hero points in PF (usually only 6+/level). Lawful acts (over time) = 1 positive karma point, Chaotic acts (over time) = 1 negative karma point. All the castes/reincarnation layers fit within a range of karma points, in a total scale of 180 points (a circular scale), at 181 accumulated points, you start back at 0 once again. So your accumulated karma points added/subtracted total at PC death determines which caste you will move into in the next life. Since you cannot be Heaven/Aristocracy, spirits go to Hell, and are NPC oni/demons...
PC death. Once a PC dies he's dead, Reincarnation (by spell), Raise Dead, Resurrection spells do not function in Kaidan. Everyone will enter the reincarnation cycle. First one's spirit goes to Yomi, the land of the dead, while your karma points, and any reincarnation/karma spells placed prior to death are summed up for karmic movement on the cycle, plus a die roll. Instead of being reborn into a new life in the cycle, the PC rolls on the Reincarnation table and the results by class, adjusted level and caste are determined. Your spirit attempts to possess a living being that qualifies to your reincarnation destination caste/class/level. A seven day ordeal called Mind Fever, where the possessed person becomes greatly ill as the struggle to his own soul begins. A roll is made to see which spirit owns the body. If you win, you wake having been reincarnated into the life of the formerly possessed person. If you fail the roll, your are now a ghost (yurei) subject to the rules of being laid to rest. If you are laid to rest in yurei form, you will begin your reincarnation process again with new rolls for new destinations.
What this ultimately means is once a PC dies in Kaidan, their soul is permanently trapped into the cycle, even an outsider (non-Kaidanese adventurer adventuring in Kaidan) dies while within the confines of Kaidan, no matter their actual deity or personal religion of choice with a predetermined afterlife destination is now subject to cosmic laws of Kaidan, trapped into an endless reincarnation cycle forever.
As an aside, there's an annual Buddhist meet of temple priests across the midwest have a meeting at a tiny town not far from me. When I used to run my graphic design/digital print studio in my hometown, I was the closest higher end graphics shop in the area so something like a Buddhist "bishop" had been coming to my shop every year. I showed her my Kaidan reincarnation cycle - and though she understood it's meaning and how it's different than the practiced Buddhist beliefs regarding it - she LOVED it. So while not an official endorsement, I don't have any guilty feelings distorting a world religion into a horror setting.