My version of GB3 goes like this.
ecto one races through new york.
It stops at a haunted mansion.
Mysterious guys in jumpsuits step out of ecto and strap on proton packs.
They go in the mansion bust some ghosts and when the dust settles we see they're RGB look-alikes voiced by the original RGB cast... ha ha.
After that We see it's all just a movie set and Venkman (murray) is the director (or consultant... whatever).
As the story unfolds we learn that all the GBs have moved on from busting.
Dana's Son Oscar is much older now.
Janine has married Egon who's a professor at a uni (

)
Ray's still at his shop, he's working on a way to bring slimer back to life. (like that machine in the casper movie.) Maybe this could be one of the focuses of the plot.
Winston's a priest, one of the higher up ones since he's personally met so many from beyond the grave.
The threat this time is indeed Satan or something like that who has come to bring hell to new york. This demon is so powerful it brings gozer and Vigo back to life or whatever they'd be and staypuft would be back too. Slimer lives with Ray at the shop and he warns ray that there's talk of an attack on earth's dimension from the ghost dimension.
I haven't worked out how the story happens yet but it'll follow a similar pattern to the last two. GB (re)establish themselves, some **** happens, and who ya gonna call?
At the end of the movie, Peter sacrafices himself and dies to save the rest of the GBs lives. I think this is great because it shows even though he's the cynical one he's still has the biggest heart and we remember that all the gb were willing to die to save new york in the first movie. This time one does. (Mostly it's just payback for making us wait so damn long for a new gb) He comes back as a ghost in the end patrick swayze style to say goodbye. I want slimer to try and slime him but slimer gets slimed by Peter instead...
Ah.. darn you murray...
Oh and the soundtrack would feature songs by eamon.