I'd love the big 3 to make a return, from what everyone has said so far, I'm very excited if they decide to use any of these great ideas!
I'm also loving the return of Walter Peck! The ghostbusters arch nemesis who they can't bust. Another idea is to work on Walter's character more and delve into his mind on why he hates the GB's so much, maybe even have him revealed that he once went to the same university as Venkman and was furious how Venkman would slack off and still get by, and he had to struggle away, and he still didn't make it. That's when he decided to join the EPA, and saw him getting famous, he decided to get even! You guys may flame me, I just thought about it when his name was mentioned and since we're talking about a long term story / plot lines etc.