Being a recurring feature in which I tell Tinseltown how to do the jobs it seems to be at a loss for - pro bono! Hollywood, steal this blog!
A lot of the Ghosbusters III discussion has centered upon Bill Murray, and how big a role (if any) he'd consent to playing. But assuming that any prospective threequel would launch a "next generation" of 'Busters, it strikes me as if anyone should appear in the movie, it'd be Ramis. Wasn't Egon the most knowledgeable and supernatural-obsessed of the four? I see him in a Master Splinter-type part, chuckling at the antics of the new gang.
As for the story, voila. I got the 'Busters go to an NYC mirror Hell idea from Dan Aykroyd-related rumors circulating online the past few years, but it's otherwise my own stuff...
All the original Busters minus Egon have retired. The new guys alone keep the torch alive, since he refuses to share his inventions, so the city and authorities need them around but sometimes only barely tolerate them, particularly when they mess up due to their general scrappy-ness. It transpires that the reason Egon is so protective of his tech is because he's found a way to shepherd ghosts and ghouls into a higher dimension of being, where they can finally know peace (and not bother anyone).
Enter a new gang in town: the Ghostshredders! Well-financed, flashy and professional, they zap ghosts with far greater efficiency than the Busters. The City embraces them...
... But before long, the gang figures out that the Shredders don't capture the ghosts, they annihilate (spiritual-murder) them. The Busters vow to bring them down, or force them to reform to more humane ("ghosmane"?) tactics.
From there, turns out the Shredders CEO is a demon, portal to hell, battles in Grid/Hell version of NYC, etc. When all hope is lost, they're saved by a mysterious spirit... it's Venkman, appearing in a super-cheesy Force-vision-esque form. Turns out all these years he's spent getting high on ganga (mixing Murray with the character here), he's been navigating other planes of existence, and has found all kinds of groovy truths. Venkman ascends to such a higher plane, the gang escapes back to real NYC, ghost-shredding is outlawed, Egon becomes a beloved public spiritual guru, and the Busters get private grants large enough to upgrade their tech. Happy end.
Take it, Columbia/Sony/Reitman/whoever! You're welcome.