I think you all know what I would like to see but, since I didnt post in a while, let's rewind it :p
After GB2, the supernatural activity never stoped. The ghostbusters were always in demand. So each city has their own franchise. Each member of the original team teach new recruits in a franchise. When the students can take care of themself, they are send in a franchise in need.
In GB2, the slime was throwed in the sky. The idea is: Vigo did that, using a spell, just before he was crush. The slime became one with the atmosfear and fall on earth each time it rain. An answer why the supernatural activity never stop.
In GB1 and 2, only a gate was closed. The enemy is still there, waiting for the opportunity. The gatekeeper and the keymaster is a spell, made by gozer, in another life. Dana and Louis were members of the Gozer's sect.
Gozer knew he won't be abble to rule the world during that time, that's why he/she made the spell. In GB1, the spell is activated and they reach to the portal. The GB closed the portal, thinking they won.
In GB2, Vigo, a knight of Gozer, tryed to open the portal again. And the same thing appened.
In GB3, the gatekeeper and the keymaster spell appen again, when the world is under a nightmare of supernatural activity. The portal will open, the GB will need to enter the portal and exorcise the enemy once and for all (a containment unit only contain the souls, you cannot allow yourself to built more and more like a nuclear waste disposal!)
In the portal, the supernatural activity will be worse. The GB will need a vehicle with ecto-deflector and strong enough to survive the portal. Why not something like we saw in Batman Begin (even if I hate it), I think the idea is better for GB (no no no, it's only a vehicle for the great danger, they will use the original car in GB3 for standart purpose. And the other franchise will have their own car).
GB3 will add new weapons but we'll see most action scene with the proton pack and trap in it.
Murray said he want to die at the beginning, let's kill Venkman on the start, in a frightening action scene versus an horrible ghost. It will throw us deep in the script (if the script is correctly made), a weird supernatural activity will pop in the sky. A race against time appen next to stop it.
Feeling weird, Dana visit Egon and ask for help. Later, same thing with Louis. The rain on the city transform vehicles in a living hell. The graves turns alive. Between the running civilians, a familiar shadow: Vigo. A magical duel follow. etc
Oh and Im tired… browse my old post, you'll read more :p