How They Should Have Made Ghostbusters III
1. Last Resort: Make It A Cartoon
This is actually an option still potentially open to Sony.
Several years ago, as word on Ghostbusters III was tediously slow, Aykroyd said that the film would probably see light as an animated venture. While it was never mentioned again and has never since been elaborated on, it was an interesting prospect, and is currently the most respectable way to make Ghostbusters III, should it ever happen.
As The Real Ghostbusters and the 2009 video game proved, there's no stipulation that GB has to be live-action to be successful. The writing can still be top-notch and the voice-acting seamless, brought to life in colourful and vibrant animation.
With a cartoon, they can write around a multitude of problems that a live-action film would present. Firstly, a cartoon is time-fluid, and can effortlessly be set in past or present; five years or 30 years after the second film. There'd be no reason to have older Ghostbusters if the story didn't require it, and so could retain their "classic" looks from the original movie.
Another reason is the passing of Harold Ramis. While never the same, a re-casted voice would be much easier to tolerate than a live-action replacement, or worse, a creepy digital stand-in. Additionally Bill Murray (who's no stranger to voice over work) might be very accepting to an animated piece, having done two Garfield movies despite being indifferent to the scripts.
Unlike the other points, none of this is impossible. Ivan Reitman and his Ghost Corps team are already pursuing multiple Ghostbusters ventures, one of which is an animated project. It's clearly something that Sony are willing to explore, and maybe they'll see sense in giving the original boys in grey one last go-around.