For me I'd have ATC2 be the classic battle with Gozer featuring the new crew but a post credit sequence to reveal Gozer wasn't the only thing to enter the dimension during the crossrip. We see three familiar faces in grey jumpsuits.
ATC3 As old and new Busters try to figure out how to fix things the variations between dimensions become clear. In their discussions the old team discovers Vigo has not surface. They can destroy the painting before he arrives. This leads to the midfilm ghost battle we all know and love. They successfully destroy the painting and Vigo in the process but fail to destroy the demon that resides in Vigo. That demon becomes the big villain of the film. So up to the midfilm battle is a call back to GB2 leaving the next hour to be an original story mixing the new and old Ghostbusters. At the climax the old busters sacrifice themselves as they are taken away with the demon to HELL! Give us the chance at the GB3 we wanted and our old busters back but keeping ATC alive.
So who were those look-a-likes in the first film that seemed like cameo appearances? Those were the Ghostbusters. But they had different names? One of the minor changes in history. Winston still worked with the dead. Venkman was on TV. Janine was a receptionist. Ray still provided thankless grueling work. And Egon was so successful in his scholastic achievements his bust was on display at Columbia. Even Dana found herself drawn to the Ghostbusters. In GB1 and GB2 we could see the effect of how paranormal activity was attracted to people and so it is in the ATC world.