Don't even have it be GB3, how about Ghostbusters The Next Generation? Kind of like Extreme Ghostbusters, but live action.
We could have Egon, Ray and Winston (since Murray wants nothing to do with it) in the first one and they pass the reigns over to a new crew. If the movie does good, then the new guys would be in any sequels.
Not saying I wouldn't prefer a third movie with the original characters, but this IS a way to keep the franchise rolling.