In all of the 3rd movie scripts or ideas I've read here are all good in their own right. However, there is a lot of repetition in the basic ideas. Many years after the Vigo incident, something else happens and the Ghostbusters go back into the fold. Sometimes, there's the original four and sometimes, there are new members. Most of these ideas have a plot that is more on the terrifying level, and they don't have too much humor in them. True, the intial idea for Ghostbusters was a serious sci-fi film, but it, too, was retooled for the early 80s screwball comedy.
Now, the most specific thing i want to address is how in each and every one of them (possibly, I haven't read every single story, just a majority. My apologies if this has been said, and I have simply missed it.) the Ghostbusters jump back into the old saddle with almost no prompting. It's almost like a reflex. They go back to Ghostbusting, no questions asked.
But my idea is this. Consider all of the crap these four men have been through in 20 years. After the first movie, they were assumed to be sued, ridiculed and basically crapped upon by everyone, despite saving the city. Now, assume that after the second, the same happened. After all, the second movie was faintly the first movie with a few differences. Or let's say that everything cooled down, paranormally speaking, and they drifted apart into normal lives.
So, a new menace to New York pops up when these guys are around 50. Naturally, the cliched line would be “I'm too old for this shit.” And that's the idea. How to convince older, tired, and uninterested Ghostbusters to strap the proton packs back on, or at least train a new group. After all, one thing I learned in my fiction writing class at the university was that to make a quest (or road trip) story to work, is to convince people to join in on the quest. Just think about how real life is. If a friend says to you, let's hop in the car and go to California (assuming you're in Virginia), you don't just get in and say ‘go.’ You would need a little convincing. Just think of how Road Trip was (coincidentally made by Ramis). At least one of the guys had to be convinced to tag along for the trip. Then the laughs will follow as you take your characters on their journey, putting them in dangerous situations that only the ghostbusters can make mundane and crack jokes at.
Well, this certainly was a long post, but I think I've not wasted my breath. Just try a small treatment based on this idea, and see where it flows. Just imagine the fun in trying to convince four 50-year old guys, who are by now so disenfranchised by everything, but are the only ones that can come to save the world.
I.M. Fletcher