I used to visit the ghostbusters.net forums , it's still a great film and for all of us that were kiRAB in the 80s, it was a major film in our childhood (still one of my faves).
ghostbusters.net said that Aykroyd and Ramis had a script, but that Murray didn't want to do a third, as he was scared of being typecast.
I also believe that Sony/Columbia weren't too eager...
Anyway, it's a shame, for all us fans...
Is GB2 reshown on E4?
gaffer