A few week s ago, Dan Aykroyd addressed the rumorssurrounding Bill Murray's involvement, or lack there of, with the proposed "Ghostbusters 3".  At the time, Aykroyd was quick to denounce the stories that Murray had shredded Aykroyd's script without reading it, saying that Murray has too much respect for him to act in such a way.  Though his interview made many wonder if they would recast the role of Peter Venkman, Aykroyd never addressed if Murray would or would no be part of the film.  Earlier this week while in England, Aykroyd finally broke the news to The Telegraph that if the movie is going to happen, they will have to do it without Bill Murray,

"The studio, the director Ivan Reitman and Harold Ramis feel there must be a way to do it, but Bill Murray will not do the movie. He doesn’t want to be involved. He’s got six kids, houses all over America. He golfs in these tournaments where they pay him to turn up and have a laugh. He’s into this life and living it. I know we’d have a lot of fun [but] I can’t be mad at him. He’s a friend first, a colleague second. We have a deep personal relationship that transcends business and he doesn’t want to know.”

If they go through with the project, the best way would be to explain Venkman's departure, or death, in the opening of the film.  There would be no successful way to simply recast the role, but to keep the role out of the film would require a complete rewrite of the script.  In the interview, Aykroyd pointed out that the most recent script, the one that Murray supposedly shredded without reading, received a great response from Columbia Pictures and the original cast, except for Murray.  Aykroyd is obviously aware that doing another "Ghostbusters" film could be amazing, or the worst film of the year if its done wrong,

“We’re not going to do a movie that exploits the franchise. The script has to be perfect. I’m the cheerleader, but I’m only one voice in the matter. It’s a surety that Bill Murray will not do the movie, however there is still interest from the studio.”

What that means is the studio sees dollar signs and they know people will still see it even if Murray isn't in the film.  The movie will not be as good if ANY member of the original cast isn't part of it, but most studios today would rather put out a mediocre film that they know would turn a profit.