The movie went through a whole bunch of rewrites! It was originally going to be set in the future and focus on a worldwide network of ghost-busters. It was the director, Ivan Reitman, who decided to concentrate the group to three New York based fighters.
Another original plot was going to focus on three ghostbusters, played by John Belushi, Eddie Murphy, and Dan Aykroyd. They would have gone on their ghost hunting exploits in S.W.A.T outfits, with wands instead of their guns. At the end of the movie, Ghostbuster franchises would have spread all over the United States!