Dank Aykroyd and Ivan Reitman have been trying to get "Ghostbusters 3" happening for so long that Generation X has already gone through the anticipation, denial, acceptance and "over it" stages of the "let's sequelize everything we always wanted another of in the 80s!" mini-mania during the entirety of its "planning."
But, whatever - it looks like it's finally really happening. For years, the hold-up was trying to convince the original cast (well, okay, trying to convince Bill Murray) to star again; but now that it doesn't look like that's happening it seems they'll be going with the pitch everyone kind of assumed they'd be starting from when they first started talking about this is in the late-90s: A new crop of young comic-actors being handed the mantle, with the originals in support roles (so, a live-action "Extreme Ghostbusters," basically.)
I'd honestly be surprised if they didn't find a way to get Murray back in at least some form - he's not exactly known for consistent behavior, and he's put the uniform back on at least twice recently for jokey cameo appearances; so if they could find a way to fit him in that only required a few hours of work and could be shot on his schedule (read: whenever he feels like doing it, wherever he feels like doing it, if he feels like doing it) I'd call it plausible. FWIW, certain early drafts of the screenplay supposedly featured Peter Venkeman returning only in a brief cameo... as a ghost.
But, yeah... apparently we've got to deal with this now. Fine.