The prompt here is simple. The concept of an entire world taking place all inside the mind has been done in movies before. In particular the movie Identity from 2003 which explores one possible scenario. Another movie with this idea is Sucker Punch from 2011 showing an entirely different concept of the world inside the mind.
A writer slips into a coma, all of the characters in their head are fighting to keep them alive.
How would this work? What would this look like? Would the characters be aware? Where does your creative thought go with this?