Admittedly, most of my knowledge of this mythos comes from “Marble Hornets”, but Slenderman (known as “Slendy” by fans”) has become an iconic, monstrous figure in his own right. He’s extremely creepy and no one knows what he is or what his agenda is. All we know is that he gives people a bad case of the heebie-jeebies whenever he shows up. The Ghostbusters may have tangled with Cthulu, but a showdown with Slendy could still give the team a run for their money.
This would be an awesome story. Pennywise is a great villain, who would be a challenge for the team. His ability to shapeshift gives us, what is essentially, every horror monster in one. Seeing as It has a tendency to reflect the fears of its prey, it could easily give us a chance to see Stay Puft sans Gozer and have it actually make sense.
Seeing as King has dropped several hints over the years that the Losers may not have actually succeeded in killing It in the novel, you could easily make it a sequel to the book. There is, of course, the option of making the story an “elseworlds” story that has no impact on the canon of either the Kingverse or the Ghostbusters world.
3. Doctor Strange
The lesser known comic hero deals regularly with otherworldly entities. While the overall theme of Ghostbusters is magic vs technology, it would be interesting to see the two join forces to battle some evil force.
Considering how powerful Doctor Strange is, it might be a little one sided, but the Ghostbusters are no slouches and have gone toe to toe with entities not too dissimilar from the ones that Strange battles on a regular basis.
2. The Shining
This is one of my favorite novels. As fate would have it, I just finished watching the TV miniseries adaptation recently. It occurred to me that the Overlook would make for a great Ghostbusters opponent.
There are plenty of ghosts, some less friendly than others; but there’s also the fact that the hotel itself is sentient and/or tied to some eldritch abomination. This would challenge the Ghostbusters without falling into a generic “the world is in danger” plot.
It would also allow the Ghostbusters to go nuts with their proton packs as any damage done to the hotel itself would, in theory, weaken whatever dark forces reside in it.
1. Silent Hill
I’m surprised no one has thought of this. Silent Hill seems like a great setting for the Ghostbusters. There are a wide variety of monsters and it would allow the viewer/reader what have you to be thoroughly creeped out. While Ghostbusters isn’t as dark or violent as the revered horror game series, it does have some rather creepy moments.
The psychological aspect would also allow us to see the characters in a new light, as the town has a tendency to use the characters’ history against them.