So, I've gathered this idea is not terribly popular then.
It's always interesting to look at the way things might have been. You could pursue this idea without drastically changing the game. For example, you could have a cadre of ancient spirits who were cast into the belly of the world in the Time Before, who are now the wicked patrons of a shadowed valley and the rude folk who live there.
You can surmise a new kind of demon from spirits or even the raksha, or non-raksha creature of the Wyld. There's room for you to distinguish spiritual ecologies without undermining the Terrestrial/Celestial hierarchy of Creation and Yu-Shan. Rather, such unique, out-of-the-mold ideas help to make those things more prominent.