First off, I would pick between this really being a full reboot or a sequel, a big problem with this movie was it didn't seem to know what it wanted to be. It was halfway between trying to be it's own thing and trying to be a continuation/remake of the originals.
Seeing as how they got the original team on board to make cameos, I think it would have made more sense to just have this be a sequel, and have a quick 5 minute scene of them being too old to do the job and passing the torch to the younger team.
Let's assume this is still paul fieg and the female busters, I'd say have them be based on the female gb's we already have from the cartoons/comics, this way we can flesh out characters diehard fans already know and love and officially canonize them in the film universe, you get to change it up and have a female centric GB team, but also still appease the fanbase. Also go back and look at if there are any ideas you can use from Danny and Harold's various scripts for GBIII so that the fans can feel more comfortable about someone else taking over the franchise knowing that the people who brought us the original films are still having atleast some input.
Despite being a sequel, tone down the references to the first movie, we don't need slimer and stay-puft, thats been done, i'd say instead draw upon the existing lore of villains from the cartoons and bring those as well into the film cannon, my choice would be Samhain.
Around halloween time, Ghostbusters re-enters the social consciousness. The song is on the radio and at parties, people are dressing up in costume, use that to your advantage to get people excited about a new Ghostbusters movie that is loosely halloween based and released around that time when people are surrounded by reminders of the first films and in the mood for something paranormal and fun. This has the added bonus of an increase in sales of dvd's and some replay value every year around Halloween time if the movie is done well.