How They Should Have Made Ghostbusters III
4. A Different Kind Of Threat
Gozer and Vigo both served as mysterious and effective villains who were fairly uninvolved with the Ghostbusters up until the climax of each movie. To give III a different flavour and structure though, we would've needed something a little different.
Emphasising and rounding out the film's antagonist wouldn't have been a bad direction. Rendering them more of a solid character, they could be fully aware of the Ghostbusters and motivated to destroy them rather than just vaguely taking over. With some more backstory and screen time with the team, it could possibly amount to a successful rivalry/revenge movie.
A different option would be not to have a central villain per se, and instead have a threat arise from a grave environmental situation; the Ghostbusters trying to heal a dimensional rift or restore some kind of universal imbalance would be nicely outlandish, and would help offer some story variety to the trilogy.