Soul seems like a pointless tag. There's no standard or possible consistent appearance for it to be actually taggable. That said, I think ghost is well defined enough not be aliased into something like soul. It'd make more sense to me to just alias soul to a more generic term like spirit (which happens to already be an existing tag) and possibly have it be implicated by ghost.
Soul would probably require clean up first. It hasn't really been used consistently.