With this news it looks like what was once assumed to be Ghostbusters for a new generation is instead a reunion for the principals involved with the first two films. Reitman, of course, produced and directed both the 1984 original and it's 1989 sequel, so it's comforting to know that he is now returning over twenty years later for the new film. Bill Murray, Dan Aykroyd, Harold Ramis and Sigourney Weaver are all, barring some kind of cataclysmic, joy-hating turn of events, returning as well (I haven't seen confirmation on Ernie Hudson). And if that's not good enough news, even Rick Moranis is said to be willing to leave retirement to reprise his role as their accountant, Louis Tully!
Given that it has been twenty years since the last Ghostbusters motion picture (the characters have transferred to a handful of other mediums since then, most notably the recent video game made by Atari), I'm sure that Columbia pictures would like the new film to function as a possible gateway for further films, but with Reitman and friends all coming back for it, we can at least rest assured that it won't solely be a passing of the torch.
Oh, and as far as the Bill Murray as a ghost rumor that Sigourney Weaver started is concerned, Ivan Reitman wouldn't comment other than to say that there are some "very cool things in the new draft."