I like the idea of motions, interactions, or types of chi instead of basic substances. But I need to get my head wrapped around how that would merge with different types of clerical channeling, or spirit types, or spell types. Without doing that there really doesn't need to be a separate wood or metal "element". Although following PF, they could just be treated like they were Hellenistic. I can imagine metal and being some combinations of earth with either fire or negative and wood as earth with water or air or positive.
It might be easiest to treat the Daoist idea as entirely separate with classes and magic system designed just for it.
Trying to keep things vaguely in line with the classic D&Disms (esp. 3.5/PF), some force/element that is the stuff that heals living/damages undead, and its opposite that damages living/heals undead are probably things I need to keep.
Going with an active negative (instead of just a lack) feels like it has a variety of aspects: there's the one that's the natural opposite of positive's aspect of life and growth (the necessary death and decay), there's the one you mention of the absolute nothingness (oblivion?), and the source that powers the undead. I can imagine a non-evil god of death going with the first of those three and being the enemy of the things that do the other two. Is that one negative energy that has a natural use and two corruptions? Or is the nothingness just what you see in that if the positive is hidden? Is positive also bad if it doesn't have the negative to balance it.
If I'm imagining spirits associated with each element I can picture the types of Djinn for the four classic elements, maybe some giant ent-like fae for wood. For negative could it be some lich like spirit? Could it be a mawing darkness of oblivion that the lich would fight against? Could it even be a corruption of something that spreads disease or is toxic or acidic?
Wood = Air.
Use the Air Element for trees and plants ... and oxygen. Also, even in European traditions, the sky can be the Tree, like Yggdrasil, and so on. In Jewish mysticism, the element of Air corresponds to the Tree of Life. In the I Ching, the hexagram of wood (double wood trigrams), talks about wind.
An Elemental Plane of Air, with lush vegetation feels right to me.
Likewise, Metal = Earth. Using Earth elementalism to forge a metal sword, is straightforward. Similarly, forming a crystal.
The confusing element is Soil, because Hellenists might assume Earth, but in fact, the Dao concept of Soil is more like the Hellenist concept of Ether, as place within which all the four elements move. When the Buddhists get a hold of motionless Soil, they use the motionless Void instead, as the nirvana-like emptiness within which the elements move, alot like eternal unchanging Ether. Meanwhile, the force of gravity permeates empty outer space, so Force and Void can be two sides of the same coin.
Soil/Motionless = Ether/Changeless = Vaccuum/Space = Force/Gravity
The fifth element Ether is the Ethereal Plane, of course. The Ether itself is made out of force. Shallow Ether overlaps and views the Material Plane. Perhaps the overlap is sensitive to soil, so things underground might be more present in the Ethereal? Deep Ether is empty perfusive force, maybe gravity maybe telekinesis.
