The concept is neat ! I think the idea could be pushed even further, but that's a nice prototype =) ! Avoiding the souls was pretty easy, maybe make it so they could follow/damage the ninja, so switching to ghost to stop them would make more sense ? Everytime I killed an enemy, I just could stay as Ninja and go away ^^'.
Also, for some reasons, sometimes I just couldn't open certain doors as the ghost... Is that normal ? The second door didn't open the first time I played, I had to reload the game ^^'...