I dont think Rick Moranis will come back, he's been out of Hollywood acting for too long.
Aye I wondered where he want for a long time (until the internet/imdb), then found out he'd basically left his career to look after his kids, something that whilst i'm sad we didn't get more films from him, makes me think quite highly of him as a person as he put family first.
I would love to see him back in Ghostbusters or virtually anything, but I can't see it happening.
A ghostbusters 3 that follows on from 2 could be good, but I doubt if any of the original cast will be back in any major way as they'e had nearly 30 years to do a sequel together but never managed to agree to it/get it done, and they're all too old to reprise their original characters in the same way (so they'd be sort of the elders, running the company etc rather than "on the ground" in the way they were in 1 and 2).