When I read this post on ghosts and zombies immediately I thought it could potentially make a really interesting story, if only I could think where to go with it.
Start with a zombie apocalypse scenario sweeping across the Earth, leading to almost the entire population of the world ending up as ghosts and... then what? The whole species having to decide "we've been wiped out, we're extinct but we're still here, now what do we do?"
Visualising plot strands of despair, would-be suicidal reactions, religious crises, some maybe want to zoom off to explore space, or persuade the ghost-species to adopt a frame of philosophical contemplation, ennui over being and observing but inability to affect any thing in the world. Interactions with the elder ghosts of humanity's past and trying to negotiate a place in / replacement for whatever society they had constructed. Maybe side-strands with the living who are still trying to survive and the question of whether they could eventually prevail, if humanity might re-emerge as a living species.
Don't know where the story would go. My own inclinations probably take it as a quest for understanding with ghost-science and exploration and a lot of people wanting to find ways to interact with the world again, or maybe to cease existence entirely. I keep being drawn to the possibility of vistas on other worlds and mysteries and discoveries that could be made out there (ghost aliens? non-ghost aliens who can or cannot detect the ghost humans?).