Those of us who finished Ghostbusters: The Video Game last year may think we've seen it all, but there's more to the story of Gozer's return than the game had to offer.  A series of concept artwork pieces created by Stuart Ng shows environments that didn't make it into the game's multiplayer mode.  ConceptArt.org has most of the pieces, while a few others are waiting at sci-fi blog io9 and Ng's own DeviantArt site.  Some such as the Ivo Shandor exhibit at the museum should be familiar, but my favorite has to be a take on the Van Horne Pneumatic Transit Station filled to the brim with pink psychoreactive "mood slime" as seen under the streets of New York City in Ghostbusters 2.  Says Ng:

"I had a lot of fun coming up with this one. Well, at least coming up with ideas of how the rest of the area looked like. In the movie, you mostly only see what's seen at the end of the train platform at the far end of the concept. I did a lot of research as to what the pneumatic subway could have looked like and what it's history was. Turns out, the station never actually existed, and was simply fiction in the movie. What was actually seen in the movie was based on an actual location, and that's what I drew a lot of inspiration and research from." 

I can't imagine how well a multiplayer environment that involved trudging through deep slime would have worked which may have something to do with why the idea went unused. Other unused concepts includes multiplayer levels from the Spider Witch's layer, the library stacks, and the architect's office.  Definitely have a look at them.