They should be champions coming into the film and challenged by something new and different. They should have the usual mortal foil like Walter Peck (but a little more intelligent that the Deputy Mayor) and an obscure esoteric monster being nasty creature to fight. I would abandon the patronizing and simplistic crap that hurt GB2 like the lack of smoking, the pandering to kids, the overuse of slime, Slimer, the Statue of Liberty walks…etc.
It's time for Cthulhu ladies and gentlemen, something EVIL, without amusing traits to bring down the threat to a goofy Vigo level. It's a movie where people will die if the GBs can't take on the challenge. Instead of Terror Dogs, it's the Hounds of Tindalos. Instead of the Titanic arriving, it's the HMS Horatio appearing in NY Harbor, cannons ablaze…
In my personal continuity, REAL and EXTREME Ghostbusters only happened as part of licensing deals for the founding team of four in NYC.