I want to make the spirit world mysterious, a thing known only through folklore and guesswork with unspoken rules so elaborate or nonsensical that no one feels like they have a solid grasp on it. Having to 'solve it' each time it is encountered will keep the Solver characters way out in front when it comes into play. I think a single journy into the spirit world will be the climax of those sub-arcs, with most encounters being more spooky brushes with the unknowable. I would like the spirit world to seem eerie.
I'd also like to keep most monsters unique - 1 kappa, 1 old tengu, 1 giant centipede, etc. Running into a 'monster' should be an unussual, memorable, and singular event.
Mysticism will be a layer apart from the factions. There are three major mystic paths known as the Three Treasures, and three minor ones. The three major paths have organized power bases, practicioners, and places of study. They are important figures to be courted and/or appeased. Each holds a sacred artifact that can significantly transform those exposed to its power (anticipate a style feat for each with steep renown requirements).
The Order of the Golden Phoenix are masters of fire who can literally undo death.
The Order of the Jade Kirin can cross the land in the blink of an eye and are masters of natural law.
The Order of the Pearl Dragon command the weather and grow to fearful size.
The three minor mystic paths are comparitively individual and private practices, with practicioners in each of the factions who may know and give each other at least a respectful nod even if their masters are at war.
The Inner Garden is a path of mental agility and personal reflection.
The Steel Way is a warrior discipline for the most unyielding spirits.
The Gabbler's Cuts trade their precious droplets of life for momentary good fortune.