that is a really fun idea with endless possibilities. gold stars for you!
some options that immediately one to mind are: - ghostly plane of existence, like purgatory. they can see and hear the others, but the others can't see or hear them. in effect, they are also ghosts for the time being. - an alternate timeline where rowan and his spirit army were victorious over the ghostbusters. everything is in chaos. post-apoc themes but not quite as soul-crushingly depressing because the good guys will eventually restore order. - another au set in the 80s and featuring the original ghostbusters as npcs. i would love to see holtz and spengler put their big, nerdy brains together. other members of the original team, like venkman, would probably be less than helpful. maybe even competitive/combative toward the future generation of gbs.