I agree, a sequel would most likely have to do with the rookie setting up a franchise in some other big city, my money would be on L.A. or Chicago or something like that. He'd probably have a team of all new guys and be in close touch with Egon who would have to be providing new equipment or Ray who would basically be doing the same thing.
The inherent problem with a sequel to the game is that without the four original Ghostbusters, you lose a lot of what makes the series funny. Yeah, it might be possible to find four other characters (or three if they make the Rookie talk) with good interplay, but they say lightning rarely strikes the same place twice.
There are other issues, but we'll cross those bridges when we come to them.
Honestly, I'd be MUCH MUCH MUCH happier seeing additional downloadable mission packs than a sequel. I know that would be next to impossible, considering that you'd need to get all the guys back together for voice work, but it could be worked around.
Please don't ruin Ghostbusters with a hairbrained ‘lets make some monay’ sequel.