My idea is pretty much the same as RedVaporFantasm`s. The only difference is i think it should be Oscar who has the inner ghost. Dana had a ghost put in her by Gozer, and when Oscar was born it left her body through him. It has stayed inside him for the right time, which is now. After it leaves his body 3 times, casuing much terror and destruction, Gozer`s gates are opened again and he returns.
The new GB`s could be one of Ray`s bookstore customers who is really into the paranormal. Another could be Egons nephew. And yet another could be another person that they decide to hire.
I figure that is just a basic idea.
…….by the way, my name is based on the ghost that comes up from the subway in the first movie…