How to write a Venkman ghost into the film? Dunno, actually. I do have an idea, though.
I think the film should start in the middle of a bust, a somewhat-more-dangerous-than-the-average bust, and either Peter is hit by the proton-stream or the ghost their trying to catch is extraordinarily lethal. So if Peter gets killed, he should be killed in the first five minutes of the film. Maybe even before the theme music is being played.
This might look a bit grim, but didn't Aykroyd say something about him having the idea that it was going to be a bit grimmer than the previous two anyway?
I do agree that if Bill doesn't want to be in it at all, they probably shouldn't make it. He should AT LEAST get a short cameo appearance (e.g. something of a nature as I just described above).
I also think - having Blues Brother 2000 in the back of my mind - that the persons possibly involved that are not called Dan Aykroyd (e.g. Ramis & Reitman)should be very critical on the script before they start the shooting and all (no offense to Dan, but I think Blues Brothers could have been made better than that…
-Killackee-