I was thinking that the forthcoming Ghostbusters game would be as close as we'd ever get to seeing a third Ghostbusters film, but Harold Ramis has confirmed at least the bare bones of recent rumours. Lee Eisenberg and Gene Stupnitsky, writers of The Office (US version) are developing a script, with Ramis, Dan Aykroyd, and Ivan Reitman consulting. Dan Aykroyd's said Bill Murray "is willing to be involved on some level".
The concept is that which was floated a short while ago, namely the original Ghostbusters being around in an older, wiser, mentor way, rather than front and centre. The purists out there (of which a tiny bit of my brain is one, or rather a bit of one) may object to changing the cast, but done right there's no reason it couldn't work properly. Any theoretical Judd Apatow involvement could only push things in a more solid direction.