9. Slimer Is A Dead Gangster - Ghostbusters
Without question the most iconic ghost in the Ghostbusters franchise is everyone's favourite hotdog-chugging, ectoplasm-leaking green slob Slimer, despite the fact that little concrete is said of his origins in the original two movies.
Shortly before the release of 2016's Ghostbusters reboot, writer-director Paul Feig confirmed that a written yet never-shot scene for the film revealed Slimer to have been a famous gangster who was arrested and executed after angrily killing a waiter who got his order wrong.
The ghostly gangster would then routinely return to the hotel where he killed the waiter and gorge on the kitchen's food supply, until one day being caught by the new Ghostbusters.
The newly assembled team would then shoot him with poorly calibrated proton beams, accidentally burning his legs off, turning his skin green, and bloating his body, giving him that iconic appearance from the original films.
Though fans will debate the dubious canonicity of this backstory given that the upcoming Ghostbusters: Afterlife will be ignoring the 2016 movie entirely, it's honestly not a terrible idea at all, and would fit in with the series' original continuity and tone just fine.