Ghostbusters II wasn't Ghostbusters I, but it's a watchable enough movie. I'm not going to hold the 1st movie against it because there was no realistic way it was going to be able to replicate that success.
The recent reboot film though, that was bad. As much as I like most of the individual actors in it when they are in their own wheelhouses, It was just a mess that felt like it was lazily cramming itself into a Ghostbusters suit to try and be bigger. The other thing that I've noted in discussing that movie with other people is that the original 4some had balance. You had Venkman as the 100% comedy character, Winston as the 100% straight-man (though he still got a couple of good joke lines in) and Ray and Egon in the middle, able to be sincere, serious, or funny as the dynamic of the scene demanded. Even Janine was a mixture of eye-rolling serious to the guys' antics coupled with sassy secretary/help gags. The reboot cast was basically all a mish-mash of different comedic types that never landed because they didn't have something to play off of. You just had the neurotic one, the bombastic slapstick one, the screwloose nutty one, the loud "having none of this" sassy one, and even Hemsworth's supporting role was mostly as the vacuous idiot.
After watching the teaser (side note, I find it funny that this video is listed in its thumbnail as a "trailer" when it's a 40 second teaser, while the Spider-Man one that released the other day was mostly called a "teaser" when it was in fact a multi-minute actual trailer.) I had an idea of how you could bridge the gap from the last film, explain away the lack of any characters you can't get the actors for (Egon obviously, Lewis probably, Peter maybe, Dana possibly?) and set them up to have a dramatic plot element to help link the new generation to the old. Then I realized it was probably far too dark for a comedy movie and mostly irrelevant given that Reitman has already penned a complete script and there's not much point in positing how anyone would write a continuation.