If you're gonna do it, do it right.
The plot should be epic, but please, don't revolve it around an established villain. That would necessitate the old hoary “They escape from the Containment Unit!” story. But if you do drag out an established villain, make sure it's because you have a good idea, not because “The fans love him.”
Human characters, on the other hand…feel free to at least reference the likes of Inspector Frump, Aunt Lois, Charlie Venkman, Cyrus Spengler and Egon's Mom, Big Ed Zeddemore, Janine's sister Doris, ect. On the other hand, feel free to leave out Professor Dweeb and the Junior Ghostbusters, unless you're allowed to run over them with ECTO-1.
Janine Melnitz should be gutsy, a bit abraisive, and yeah, she's completely stuck on Egon. In the RGB/EGB universe, at least, that embarrassing scene with Louis in Venkman's apartment was just a passing fling. Forget all about “Mommy Janine”. In the greater arc of Ghostbuster history, she's far more important than Dana Barrett, Louis Tully, and Walter Peck ever will be.
Slimer should be there, but take a cue from EGB: restore him to babbling pet who spends most of his time eating and annoying Venkman. He's not smart, he's not brave, he's certainly not the Fifth Ghostbuster.
Find someone other than Dave Coulier to do Venkman. Even with good scripts, he still sounds more like Bill Murray in Caddy Shack than Bill Murray in Ghostbusters.
Throw piles of money at J. Micheal Straczynski to at least participate on the script. This is the guy who wrote many episodes that, to some fans, are as definitive to the property as the first movie, and better than the second.