Had a read thru the design doc, its a good start, and heres a couple of initial thoughts:
State 1: Still not alive, but much closer to being so.
State 2: The Normal State. Not living, but not dead.
State 3: Still not dead, but much closer to being so.
Why dont you make the states more clearly defined, but perhaps put a time limit on them. So from Ghost form, he can become fully alive or dead but only for a limited time or under certain circumstances. That way you can more clearly define what he can do while in each state.
Wall Jump
The player can wall jump when against a wall. This can be useful for getting out of tight places, or simply adding some more different kinds of movement.
My first thought is that a wall jump doesnt really suit the ghost character. Which leads me to thinking that the entire movement of the character needs to be well thought out. He could have very low friction and glide just above the ground. Jumping is more propulsion into the air with a jetpack like effect, as you mentioned with the z key holddown, being able to control his descent would be good, but basically set his jumping up like a smooth jetpack.
World Overview
I like the idea of an abandoned area of the forest with old rusting technology and supernatural feel to it, you can create some pretty unique environments, characters and story.
SummaryBit of a blab, but basically what I see that will make your game unique is the 3 defined states (and the abiltiies/powers that go with those states) and character movement, so they should be the primary focus of development/testing.
Goodluck