I wouldn't get my hopes up for the original cast, at least not in a starring role. Here's what the Reuters
The original filmmakers, including director Ivan Reitman, and cast are aware of the project and involved in its development. Some original cast members might be involved, but not in central roles.
Well, I just hope they give Ernie Hudson really equal status this time instead of treating him like an afterthought.
I used to be somewhat disappointed in Ernie Hudson's smaller role in the films until I saw an interview with Sigourney Weaver where she said she saw her role in Ghostbusters as like Margaret Dumont in the Marx Brothers films. As I thought about, the parallel works out fairly well with Bill Murray as Groucho, Dan Akroyd as Chico, and Harold Ramis as Harpo.
Unfortunately for Ernie Hudson, he's stuck with the Zeppo part. So in that respect, it makes sense.