Geoff Manaugh, creator of BLDG BLOG (and occasional contributor to io9), is known for his spellbinding, science fictional interpretations of architecture. Inspired by the building that housed the old Ghostbusters headquarters (pictured), as well as the forbidding New York telephone exchange building, he's come up with an idea for the movie that involves a haunted telephone system and angelic neurobiology.

It's worth quoting at length from his post:

It's 1997. NYNEX is on the verge of being purchased by Bell Atlantic, after which point it will be dissolved in all but name.
But all hell starts breaking loose. Pay phones ring for no reason, and they don't stop. Dead relatives call their families in the middle of the night. People, horrifically, even call themselves – but it's the person they used to be, phoning out of the blue, warning them about future misdirection.
Every once in a while, though, something genuinely bad happens: someone answers the phone... and they go a little crazy.
Thing is – spoiler alert – halfway through the film, the Ghostbusters realize that NYNEX isn't a phone system at all: it's the embedded nervous system of an angel – a fallen angel – and all those phone calls and dial-up modems in college dorm rooms and public pay phones are actually connected into the fiber-optic anatomy of a vast, ethereal organism that preceded the architectural build-up of Manhattan.
Manhattan came afterwards, that is: NYNEX was here first.
It's worth recalling, in fact, that NYNEX – at least according to Wikipedia – actually stood for New York/New England, "with the X representing the unknown future (or 'the uneXpected')." It's like Malcolm X's telephonically inclined, wiry cousin.
So the phone system of Manhattan – all those voices! all those connections! leading one life to another – starts to act up, provoked by its dissolution into Bell Atlantic... and the Ghostbusters are called in to fix it.
Fixing it involves rapid drives from telephone substation to telephone substation, from library to library, all while Dan Ackroyd's character keeps receiving phone calls about a family crisis... his ex-wife is calling... his dad is calling... they're urging him to stop this whole, crazy Ghostbusters business... He starts acting funny. The voices on the phone say strange things. They call at strange hours. He feels kinship with public pay phones; they sometimes ring as he walks past. He tries to call his family back – but they're not answering.
Harold Ramis starts to suspect something.
In the background there are shadowy figures called out to fix transmission lines – but they are actually wiring something up... something big...
The whole movie then leads up to the granddaddy of them all: an electromagnetic confrontation inside the windowless, Brutalist telephone switching tower at 33 Thomas Street (rumored haunt of the ghost of Aleister Crowley).

You can read the rest here, where he goes into more detail that makes the idea sound even more fascinating. I love the idea of the telephone system being haunted, because at this point telephone systems are so ancient that they have begun to seem spooky. That is, they are as ancient and spooky as the NY subway system, which as you'll recall had a role in previous Ghostbusters movies.

I'd love to see a movie like this get made, but I'm dubious about whether it could fly in Hollywood. I could, however, imagine it as a plot arc in Supernatural - if it were the phone exchange in Lawrence, Kansas.

via BLDG BLOGhttp://bldgblog.blogspot.com/2009/06/nynex-embedded-angel-of-new-york-city.html

Writing to the Chicago Tribune despite the apparent loss of his Caps Lock key, Ramis revealed that at least a couple of the original cast are definitely onboard for the third movie:

yes, columbia is developing a script for GB3 with my year one writing partners, gene stupnitsky and lee eisenberg. judd apatow is co-producing year one and has made several other films for sony, so of course the studio is hoping to tap into some of the same acting talent. aykroyd, ivan reitman and i are consulting at this point, and according to dan, bill murray is willing to be involved on some level. he did record his dialogue for the new ghostbusters video game, as did danny and i, and ernie hudson. the concept is that the old ghostbusters would appear in the film in some mentor capacity. not much else to say at this point. everyone is confident a decent script can be written and i guess we'll take it from there.

The same acting talent as Apatow's other movies? So that would mean Paul Rudd, Jason Siegel and the by-now ubiquitous Seth Rogen for the main roles, I guess... but who will be the new Annie Potts?

Harold Ramis confirms 'Ghostbusters III' [Chicago Tribune]