I think the original was a fluke. All of the right things, all of the right ingredients, in all of the right places at all of the right times. In many ways, it probably shouldn't have worked, yet it did and did it perfectly.
After all, look at Ghostbusters 2: sure you may have a soft spot for it, sure it might be good with some flaws, but it never captured the success of the original.
Ghostbusters 3 technically exists with the video game, but even then, it had to repeat what has been done before. And it works like that because...well, it wouldn't if we saw the actors in their actual ages. GB3 would've been up there with Indy 4 and Blues Brothers 2000.
The only things that worked past those movies were (with ALOT of exceptions from one) are The Real Ghostbusters cartoon, and the Extreme Ghostbusters animated series. I kept saying with this reboot that they should look at Extreme because that worked with what was done before, and it helped create a new series out of the old and into the new. Hell, have Ernie Hudson and Dan Aykroyd in it as those helping in shaping up the new team, with Winston as the drill Sergent, and Ray still being the heart, while being torn about doing this alone without Egon at his side. Peter, if Murray still wanted to do it, could just be a cameo along with Weaver if she wanted to do the same.
I don't know what they could do at this point for the sequel to the reboot if the sequel is iced as they said. As of now, the reboot is sorta 'there". The sequel could be better than the first, but after all of the crap from the internet to the studio over this movie, will people really, REALLY, be wanting to try another film again?