I came up with a simple plot that was different from those in the other two GB movies: There's just one haunted house, which seems like an easy bust to the guys, and it is. But when they keep getting calls about the same house, they think that something's up and they start their investigation. And then, unexpectedly, the house releases a surge of energy that alters and mutates the city. In to what, I don't know yet. What do you think?
As for the guys being older and not looking as good, just have them recruit new guys. It could still be a great movie, even without the same cast.