Let's just chalk this up as a rumor for right now but this story arc could actually work if they do end up making Ghostbusters 3. The question is will Bill Murray do this film or with a cool story like this do they even need him now?
Much like Steven Spielberg did with Indiana Jones and the Kingdom of the Crystal Skull (where Mutt Williams is Indy and Marion's love-child), Ivan Reitman is looking to kick-start a new franchise with a ghost bustin' love-child of their own.
Early word from an insider is that the (hinted at) love affair between Dr. Peter Venkman (Bill Murray) and Dana Barrett (Sigourney Weaver) has a much bigger impact on the franchise than just an angry Vigo (Wilhelm von Homburg).
From what we're told, it will be revealed that Dana's son Oscar is in fact Peter's child as well. Furthermore, Oscar is now 21-ish and ready to take the reigns of his father's ghost busting business!
It's an important plot point as it gives a lot of weight to the actual story, and takes it from being "just another sequel" to having a real purpose (so long as they don't pull an Indiana Jones IV)."