Why not have the new movie be about HIS grandchildren. Spengler was basically just a gag character. They could have had Murray die and be a ghost, that way he could be funny with new material and CG Egon would return at the end with the other GHostbusters and just say "Yo."
shareWhat they REALLY need to do is bring in Ryan Reynolds as Venkman's son, Oscar, since Reynolds is the closest thing we have to a "new Bill Murray."
shareMovie was sort a love letter to Harold Ramis so it would make sense for the new characters to be his family and having him be a ghost he wouldn't have to say much without it being inconspicuous.
Probably didn't make it Venkman centric sense Bill Murray didn't want to do a third movie for years.