Taken from Cinefex Magazine #40:
"For the sequel, Dan Aykroyd once again wrote the initial script--a story in which Dana was kidnapped and taken to Scotland where she discovered a fairy ring and civilization underground. "My first draft was really too far out," Aykroyd reflected. "It was probably too inaccessible, though I thought at the time I wrote it that it was the direction we should go in. I wanted to leave New York City behind because I thought we had done that. But New York is really the greatest arena for our kind of ghost story, and staying in the city gave continuity to the second film. One idea that did stay in our script was the notion of having things occur underground. We went skyward in the first film--up to the top of a skyscraper--so I thought for the second one it would be nice to see the underbelly of the city. But my original concept for going underground was different. It involved a pneumatic tube two thousand miles long that they traveled in for three days. It was like a primitive mail chute."
The reason for the nickname "The Last of the Ghostbusters" was Bill Murray that said in Starlog Magazine #140: "It's not going to be called Ghostbusters II. We'll burn in hell if we call it Ghostbusters II. I've suggested The Last of the Ghostbusters, to make sure there won't be anything like a Ghostbusters III."